Smile makes us forever young

"Frown and you frown alone," , "but smile and the whole world smiles with you." The whole world? Not just any smile will do, though. Here's how to develop a genuine, infectious smile that can make a bad date turn good, seal a business deal, and help you make friends wherever you go.

[edit] Steps

Practice good hygiene. If you've got bits of food stuck in your teeth, people may indeed say you have an infectious smile, but they might be talking about actual infections. It should go without saying, but regularly brushing your teeth and making sure your breath is fresh are prerequisites to a good smile. If your teeth are stained, visit the dentist for a polishing or use a whitening product, and take care of your lips to prevent chapping. When you smile people will inevitably look at your mouth, so following these considerations will help you make a better impression, and, more importantly, a healthy mouth will make you feel more confident about smiling.
A smile lights up your face

Get comfortable with smiling. Many people are a little nervous about smiling. They don't think their smile looks good, or they think that smiling makes them look unprofessional or vulnerable. It's true that a smile does show a little vulnerability, but that's part of what makes it so powerful. No matter how professional, intelligent, or in control you think a serious face (or worse, a frown) makes you appear, people almost always respond better to a smile.
Think happy thoughts. The easiest way to a great smile is to be happy. You can be happy all the time, but not everybody is; when you are not, you can think happy thoughts instead. Think about something or someone that you care about, or think about a joke that you just find hilarious. Remarkably, when you're feeling down, smiling can help cheer you up, even if you have to coax a smile out at first.
Smiles are contagious.

Smile with your eyes. When we think of smiling, we think of the mouth, but the eyes may actually be more essential to a warm, genuine smile. Smiling with your eyes is difficult to describe--in general your cheekbones lift slightly and your eyebrows dip a little--but when you see it, you know it: it's that look of your eyes "lighting up" or "twinkling." To get a feel for how to make your eyes smile, get in front of a mirror and practice smiling, but concentrate only on your eyes. You may find it helpful to cover the lower part of your face with a piece of paper. Play around with it a bit, and you'll find that you can make your mouth smile when your eyes aren't smiling, and you can also smile only with your eyes. When your eyes do smile, remember how it feels, which muscles are working and how. With practice, you'll be able to smile with your eyes at will.
Develop your smile. Don't particularly care for your smile? It's a common complaint, and while it's true that any smile is better than a frown, it's also true that some smiles look better than others. What's your best smile? One way to find out is to look through pictures of yourself. Many people who consider themselves unphotogenic are plagued by bad smiles in front of the camera, but just about everybody has at least one picture in which they just look great, with a perfectly natural, contagious smile. Find that picture and focus on what your face is doing. Then practice in front of a mirror until you get it just right. Keep practicing, and pay attention to how that great smile feels, so that you'll be able to replicate it without looking in a mirror. Soon it will become second nature, and you'll likely find that you'll look more photogenic in your next batch of pictures.
If you want to improve your smile, look at pictures of smiles that you believe are beautiful. Also, think good thoughts and remember that you are beautiful (inside and out). Your smile is bound to look better if you feel good about yourself!

[edit] Tips

You don't need perfect teeth to have a perfect smile. Not everybody has perfect teeth, and that's okay. Beautiful teeth are nice, but a great smile doesn't depend on Abercrombie & Fitch teeth. That said, if you feel uncomfortable about your teeth you may be reluctant to smile, and that's no good. If that's the case, either learn to appreciate your individuality or look into cosmetic dentistry procedures.
Begin your smile with your eyes. If you find that every time you try to smile with your mouth, you look like you're faking it or, worse, like you're in pain, you may find it useful to look in a mirror and smile only with your eyes. Once your eyes are smiling, they tend to pull your whole face (including your mouth) into a natural, beautiful smile.
You've probably heard of "contagious" or "infectious" smiles. There's a reason for this. When you smile, it does tend to make others around you smile. Even in the most stressful times, a roomful of smiles can brighten everybody's mood. Either that or make someone seriously pissed at you, just because you are smiling when they are talking about someones funeral...
Ever laugh hysterically about something you've just seen? Now, think of that instance again will help fuel a full blown smile.
A lot of men feel uncomfortable about smiling at strangers, especially at other males. If that is the case, then just offer a casual "Hi", or "Hey man!", or "What's goin' on?". It works just as well, and feels less awkward for a lot of guys.
Try a light smile: dont show as much teeth as you would when you would be laughing-its the best smile for flirting-light and seductive.
If you have a tooth that pokes out a bit at the side, as many people sometimes do, try to get your lip to go above it so your lip doesn't catch on the tooth when you smile.

[edit] Warnings

A fake smile can make you look phony or even dangerous.

A fake smile can make you look phony, nervous, or even dangerous, so don't just try to put on a smile without first practicing or putting yourself in a happy state of mind. Body language reflects our innermost feelings, and it's often easier to change our attitudes than to try to make our bodies communicate emotions we don't really feel, like if you are very happy it will show through your teeth. If you smile a huge smile when someone looks at you, you will look like a freak. So keep a tiny smile and not a big one.