Strengthen your abdominal muscles and lose body fat. The concept is simple, but putting it into action is not. It will take dedication, willpower, patience and time to get a six pack but in the end, the effort is well worth it. You can have the most toned and defined abs but they won't show through if there's a layer of fat over them.

[edit] Steps

The simple step-by-step process to gaining and maintaining six pack abs is as follows:

Build Muscle

Do Crunches. Lie on the floor (with or without a mat) and cross your arms in front of your chest. Bend your knees up as far as they go. (Do not place both hands behind your head. Placing both hands behind your head can cause lower back problems in the long run as it places unnecessary stress on that part of your body from pulling on your head and neck.) Another alternative is to cross your arms in front of your chest and lightly place your finger tips behind your ears, without pulling on your neck or ears to help raise you off the floor. Draw in your abdomen towards your spine while inhaling through your nose. Now raise your shoulders (upper torso) towards your knees, using strictly your abdominal muscles. It is very important not to lift your entire back off the floor, as this can cause back strain, and the extended movement does not help you develop six pack abs any faster. The most important part of the crunch is the initial flexing of your abs as you lift your shoulders off of the floor. As soon as you begin lifting off the floor exhale through your mouth, ending with a gasp once your shoulders are off the floor. Then pause for a second once you are at the top of the crunch and exhale the last bit of air from your diaphragm while flexing your abs. The proper breathing and flexing make all the difference. Now lower back down slowly and controlled while inhaling through your nose, just until your shoulder blades touch the ground. Do not let your head touch the ground and repeat.
Do Sit Ups. Lie on the floor, feet on the floor, knees up and fingers placed behind your ears or hands crossed on your chest. Sit all the way up, lifting your lower back off the floor along with your shoulder blades.Lower yourself down. Repeat. Once this becomes relatively easy for you (i.e. you can do a quite a bit with ease) start adding more challenges. Find an incline bench or do these on an exercise ball. Once you "graduate" from that, do weighted sit ups. Hold a weight on your chest while you do these. As these become easier, hold heavier and heavier weights. You might also try lifting your feet off the ground while doing the sit ups or alternating the leg in the air, like pretending to pedal while sitting up. If your hands are behind your head, bring your left knee up to touch your right elbow and then your right knee to the left elbow.
Do Leg Lifts. Lie on the floor, legs straight out, hands at your sides. Lift your legs straight up (not bending your knees at all) until they're at a 90 degree angle (or close). Lower your legs and repeat without letting your legs touch the floor. For more challenge there is equipment at most gyms that will allow you to raise yourself up using your arms as support and dangle your legs. You can perform leg lifts there too. If you're using this piece of equipment, you can make it easier by just raising your knees to your chest. It's more difficult to raise your legs to a horizontal position with your legs straight. This helps firm up the lower abdomen. If you're truly a monster, try doing leg lifts with a medicine ball hanging from your feet. Or hang from a pull up bar and raise your legs in front of you all the way up to the bar. Still too easy?...
Do Jackknife Sit Ups. Lie down flat on the floor. Place your hands on the ground to your sides for balance; you can pick them up as you get used to the movement. Simultaneously raise your knees and torso so that your knees and face meet on an imaginary line extending from your pelvis to the ceiling. You should be able to kiss your knees at the top of the motion. Your legs will naturally fold bringing your feet towards your hips, much like a jackknife. Lie back down (i.e. "spread out") and repeat. Place a weight between your feet when you think you can handle it. What's that? More?
Do V-ups. Lie on the floor, legs straight out, hands on the floor but this time extended out over your head. Simultaneously raise legs and torso. Don't bend your knees! What kind of V would it be if you bend your knees? Reach with your hands toward the raised feet. Touch your feet if possible (will require some flexibility). Relax, return to starting position and repeat. Add weight between feet to match your taste.
Static Hold and Side Statics. Put your body into the push-up position but with your elbows on the floor, and your whole body flat. This position is known as the static hold position and it trains your core (including your abs) to hold the body in place which is the real purpose of your abs. Hold this position for as long as possible, but you should be aiming to start off with at least 45 seconds, while seasoned ab workers known to achieve over 20 minute static holds. To perform the side static hold roll onto one side of your body and lift into the same position as before, but this time only one arm will be on the ground with the other arm pointed straight up the air and your non-weight bearing leg resting on your bottom leg. Once again, hold this for as long as possible
Train Your Oblique Muscles. It's not as important to work on your oblique muscles at first, but eventually you'll want to start working these too. These are the muscles to either side of your stomach. There are multiple ways to do this and anything that includes twisting your torso against a resistance counts. There are twisting machines at gyms, you can twist while you do sit-ups, you can do side bends, you can twist side to side with a medicine ball in hand, etc. Be aware though, that many beginners tend to have weak obliques compared to their abs (it simply isn't used as much in daily life) so go easy on the sides at first.
Know that since it is, literally, the center of your body there are many other tricks you can use to train your abs, and some of the other movements will more or less involve your abs. Including every abdominal exercise in existence would make this article painfully long and new methods are being developed constantly. Now that you've made up your mind about a washboard mid-section, get creative! Find new ways to crunch, bend and twist in your daily life. Some possibilities include:
- Use a stability ball. Do your crunches on the ball to introduce instability to your workout, which will improve your balance too. There are also lots of core exercises that can be done with a stability ball.You can also use a small bubble use for physical therapy.
- Duck and twist during your daily routine. Reach with your left hand to things on your right and vice versa. If you feel like turning around to face something, see if you can do it with keeping your hips in place and twisting at the waist (warning: awkward when talking to other people, use only against inanimate objects). While walking or standing, pretend that something is coming toward you and you have to duck to get out of the way. Do this as often as you are comfortable or at times when it won't look weird. You can bend forward from the hips or, if you're really into it, bend at the knees too and really "sink" out of the way. Caution: If you have lower back problems, you could injure yourself.
Do the Scientific Best Abs Exercise Researchers at the San Diego State University, found that there are certain abs exercises that triggers most muscular activity. The winner was the "bicycle maneuver":
- Lie on the floor; make sure your lower back is pressed on the ground.
- Take your hands behind your ears. Then bring up your knees at a 45 degree angle.
- Start doing a bicycle pedaling motion. Touch your elbow with the opposite knee (right elbow with left knee and so on, alternating)
- Breathe relaxed and evenly throughout the whole exercise.

Lose Fat

Lift Weights. The more muscle your body has, the more calories your body burns, even at rest. Many people are afraid of getting huge due to weight lifting but the fact is you won't!, Many huge body-builders out there have been body-building for years to get huge. If you just do cardiovascular exercises without lifting weights then you will lose the muscle mass, you will also feel weak and it will take longer to lose that fat. So, to get rid of that fat quickly lift weights combined with cardiovascular exercises. Running, biking, swimming, stair climbing, jumping rope, tennis, volleyball, dancing, squash or any other activity that gets you moving and keeps you moving is a great way to burn fat. However, a cardiovascular workout should be performed for at least 20 to 30 minutes to burn fat. Prior to this your body will simply run on its stored energy, which results in minimal fat loss. Cardiovascular training should always be done after your weight lifting workout. While lifting weights, you use stored, (carbohydrate) energy, thus, by performing cardiovascular exercises after weight training your body will have less stored energy to use and go straight into the fat storage for its energy use. The overall best way to shed fat fast is to do cardiovascular exercises right when you wake up. Your body will search for energy to use, and when it finds nothing in your stomach, it will go directly to your fat reserves for energy.
Keep Metabolism Steady. Eating one small meal (such as a potato, a salad, etc.) every three hours that you are awake will not speed up your metabolism, rather, it will keep it going. Your metabolism goes and slows with your food intake and eating something small every three hours will keep that metabolism burning calories and will help you lose weight. A jar of nuts in the office will do just fine, grab a handful when you feel like it. Most fruits can last for a couple days without refrigeration, and if you have a refrigerator at the office, make use of it.
Eat Smaller Dinners. Cut down on the size of your dinner. If you're hungry, snack on fruits or other healthy, low calorie snacks. Large dinners tend to hurt a fat loss process because most people aren't very active after dinner. This is the basis for advice along the lines of "don't eat anything within N hours before going to bed". The claim that your entire dinner is stored as fat isn't entirely true (the process is more complicated than that) but the fact you don't move after dinner is enough to hurt your cause. You can offset this by eating a larger lunch or snacking healthily before dinner. Fresh fruits or vegetables are excellent choices for curbing appetite not to mention other health benefits. A handful of nuts might do the same. Drink a large glass of water before sitting at the table.
Eat More Fiber. Most people don't get enough fiber. The recommended amount is actually not that much if you eat a healthy diet. "Fiber foods" include whole grains, fruits and vegetables, and nuts and seeds. Other options are fiber supplements.
Eat Breakfast. Many people skip breakfast because they don't have time for it. Keep this in mind: You don't have time to skip breakfast, it's simply too valuable to skip. The fact that skipping breakfast messes up your concentration and other mental functions is beyond the scope of this article. The harm of skipping breakfast from a weight loss perspective is it makes you eat a huge lunch since your body hasn't had anything in the past 12 (or more) hours. When you eat a huge lunch you get that after meal drowsiness so now you're both unproductive and inactive. Cereals don't take much time to prepare and consume, and most of them are very healthy nowadays. If you are extremely pressed for time, consider grabbing a box of breakfast bars or a smoothie and throwing one in your bag when you leave for work or school. Some breakfast bars out there are also excellent sources of fiber.
Drink More Water Every Day. Many places suggest 8 cups (2L) of water a day. It also depends on your weight (1L/20kg would be good, more if it's hot or you're ill). It sounds like an absurd amount of water, but actually, you can drink tea without sugar to make up some of the quota. In fact tea, especially green tea, rev up your "metabolism" (actually cause a temporary increase in calories burned).
For fat loss purposes, it is important to remind yourself that thirst is a much weaker stimuli than hunger. If you consistently feel hungry after meals, don't immediately think that you need to eat more. You may simply be thirsty!
Needless to say there are many benefits of water. The human body is anywhere from 55% to 78% water depending on body size.

[edit] Top Tips

Motivation is the key. Regardless of aim, having the right attitude towards your motivation is important. If your motivation source is purely the result, you will likely not last very long. It's important to enjoy the exercise, the immediate and long term benefits and to incorporate enjoyable activities into your exercise routine. And remember, your significant other can also benefit from mutual exercise activities.
It may help to use a record-keeper or plan book for your exercise program. Exercise journals are fantastic for this purpose because as you enter daily information in the journal, you're putting together a complete record that can be referred back to in the future.
If you feel you need to, ease into the changes in your lifestyle. Start eating breakfast. If you have difficulty with this, start with a couple glasses of water or a large cup of coffee right when you wake up. It'll go down easy and prepare your stomach to get back to work. Cut the amount of dinners little by little. For the exercises, start slow and increase pace and/or repetitions as you are comfortable. Attempting lifestyle changes in a drastic, sudden manner can backfire. Do what you can but don't sell yourself short.
Make a photo diary of your body but don't take pictures too often (once a month is fine). Don't study your pictures intensely since you might not get motivated if you don't see a difference. If you are consistent in your diet and exercise, you should see changes monthly. Keep in mind that you won't see changes in the mirror everyday since they will be very small improvements.
Unless you are watching your weight for another reason, ignore the reading on the scale. You may be decreasing your body fat but your weight may stay the same (or even increase) as you build muscle. Muscle tissue is denser than fat so it will be heavier than the same volume of fat. Water weight can also vary your readings by a surprisingly large amount.
Try to keep a steady eating routine. It will throw your body off if you skip a meal and starve yourself only to stuff your face later. Avoid such situations as much as possible.
For added variety you may look into using a swiss ball and/or a medicine ball during your ab workout. Doing crunches on the swiss ball will improve your balance, or you may use it to get a little more flexion in your abs. The medicine ball has many different uses, the simplest one being added weight.
If you are already skinny, but simply don't have the abdominal muscle that you want, passively exercising while doing other things can help a great deal. If you spend a long time in a sitting position, try continuously clenching your stomach. Get in the habit of doing this for hours at a time and you may be surprised at its effectiveness.
Try to make an effort to make your abs strong while you are young because the abs are one of the last muscles to atrophy, so you may not have to worry about them when you are older if you do a good job while you are young.
Try some stress relieving activities, such as yoga or meditation. Built up stress causes an increase in your weight.
You should make working out a priority like keeping a job, you don't want to miss and not get paid.
500 calories' is the calorie deficit a day needed to lose a pound of fat per week. This may be from eating less or hitting the gym.

[edit] Warnings

Most, if not all of these exercises are not recommended for people with lower back problems. There are many other ways to properly develop six pack abs without putting so much strain on your back. If in doubt or experiencing lower back pain, talk to a physical therapist for proper ways of performing the routines.
This exercise regimen will only work for a select few individuals. For some people, this exercise advice will actually provide the opposite desired effects. For example, for people who have a few extra pounds around the belly and start this from scratch. They are actually more likely to look larger in the belly for a period of time since they are putting on muscle beneath their fat at first. You may want to begin for a significant period of time by doing more cardiovascular exercises to lose fat all over the body. Doing specific muscle exercises, unfortunately, does not remove fat from that specific area of the body. Additionally, focusing solely on the abs can produce an imbalance in the body and may cause back problems where there were none before or exacerbate any existing problems
As with any exercise program, consult your doctor before making major changes in your workout routine if you have medical problems. Increase loads slowly. Being eager is great but setting unrealistic goals isn't going to help you in the long run.
Don't starve yourself. If you go hungry for an extended period of time your metabolism will slow down. The body, thinking that no food is available tries to make the best of what it has and goes into power-saving mode. Whilst on 'power-saving' mode, the body will supplement its carbohydrates (being burnt for energy) with the proteins in your muscles. What this means is that you are in fact eating back into that six pack just to try and make it show through.
Fiber also acts like a mild laxative. Be careful if you eat a lot of fiber. It can also cause some gas if you're not used to taking in so much fiber. If you increase your fiber dosage by a lot, increase your water intake too. Excessive fiber with little water can result in digestive problems.
Habitual snacking on unhealthy foods is often done almost unconsciously. Many people do this and don't realize how many calories they take in. They insist they eat very little (which is true during their major meals), but they don't realize that their snacking is making them gain weight or stopping them from losing weight. Avoid buying high sugar snacks and instead buy healthy, low calorie snacks. A way to prevent unhealthy snacking is to write down everything you eat in a day. As you watch the list grow you will be inclined to put that donut down.
Be wary of ab machines that you may see on TV. Many of these are in fact rip offs and you are more likely to stay motivated if you incorporate an ABS workout into a regular gym session(you are less likely to get sick of it). You can get a great ab workout just by using the floor. Remember that it's not the exercise that will give you the six pack. It's the combination of strength exercises and fat loss. Playing more athletic sports, weight lifting, and cardiovascular exercises are a million times more effective for fat loss than using an exercise machine. If you feel you need to use those machines, a gym membership will likely suit you much better.
Supplements can be useful, but they are just what they advertise themselves to be; supplements. There is no magic pill to make you lose weight with no work. There are some supplements out there that can help you lose weight by increasing your metabolism or by helping you curb your appetite, but you still have to work. Most tend to be "snake oil" and will not really give you any results while costing you great deals of money. And remember, even if they DO help, all supplements still require the consumer to watch their diet and increase their exercise. A simple multivitamin or mineral pill is likely all that most people will need.
Sit ups can be bad for your back and neck. They have caused many lower back problems and are not recommended by most doctors. More and more K-12 schools are phasing sit ups out and switching to crunches. A good alternative to sit ups is to lay on your back, lift your feet in the air as high as you can reach them, and lift your butt off the floor instead. The best results come when you do them slow and tighten your abs (and even butt). When you lower your legs do not let your butt touch the ground completely, then repeat. This exercise conditions, tightens, and hardens all the same muscles as a sit up when used with crunches as well.
Don't neglect warm-ups or you may find yourself pulling muscles and long-term pain when you start doing this method!
Opinions on weight loss and muscle training routines are like noses - everyone has one but they're all different. In the end, you should find something you enjoy and that you will stick for life. Over time, you can modify it until you get the results that are right for you.