Guys often try way too hard or don't give any effort at all. This how-to (written, of course, by a girl) will let you know what a girl is really into.

[edit] Steps

DON'T CHANGE WHO YOU ARE FOR HER! Be yourself, and if she doesn't like you for that then she isn't right for you. The last thing you need is a girl who's not interested in the real you. No matter how gorgeous or fantastic she is - it isn't worth it!
Always smell fresh and clean.Bad breath will be something you regret. Brush your teeth, style your hair, and dress well enough that it is obvious to her that you know how to take care of yourself (FYI - even this starting point will rank you way past most of the guys out there)
Wear cologne. Not too much, but enough so that the girl will recognize it when in proximity of you. When picking out cologne, try to stick to the same type and try to avoid common scents - but don't go with an scent that is too unfamiliar. Get a few tester bottles (these are free) and wear each one on separate days. You are doing this so you can figure out which you like the most and which works best for your chemistry. You may do different variations of scent as well. Using natural extract oils, such as lavender, vanilla, and the like will give you a unique smell (most natural foods stores will have these items) at a lower cost.
Listen when you engage in conversation. Girls can tell if you are distracted or if you don't really care about the subject. They know these things, regardless if you think they can't tell. By listening, you show that you genuinely care about her and her interests (plus, everyone loves talking about themselves, even more, they love people who want to hear them talk). Also, when engaging in conversation don't second guess yourself when you speak. There will be a point when you need to discuss your interests and preferred activities. Do not ask for their approval, but keep the dialog about your respective self short and never incriminating nor negative. This will generate both "mystery" and a sense of decency of your persona matched with beautiful confidence and conciseness.
Pay attention to the details. When you're dressing, do you look like every other boy? Flipped up collar, same shoes, same look, same talk. Time to change, baby. Find something custom to you, whether it is how you wear your belts, your shirts, hat(s), any form of jewelry, whatever! This shouldn't be overdone. Really be your self, what we mean here is, be who you are. Confidence is key, but play a LITTLE hard to get! It should be a reflection of yourself and your difference among "all the other boys." Don't be scared to stand out. Women will associate your attention to the details with how serious you are about them.
Be sure to notice the little touches to dress and style that she may have added. Pass a compliment that is one of a kind and you may have just struck the strings of her heart, example: "wow, I love the pink with green eye shadow you use, it's so vivid." She took some time doing that, so you'd better take note.
Pay attention to your hair and eyes. These are two of the most important features of all when a girl is checking out a guy. Hair should be clean, properly conditioned, but it's also very hot when a guy's hair has a messy style.[Messy as in clean, but not brushed] As for eyes, there really isn't much that you can control. Contacts are an option, but aren't natural and can be misleading.
Make eye contact, hold for 4 seconds, look away. Repeat. But looking at them directly in the eyes during conversation and not jolting side-to-side will gain their trust and probably their respect, and should be done for maybe 9 seconds. But, don't focus on counting. Just do what is confortable for her. Show her some love boy!
Keep in shape, but don't obsess. It probably wouldn't hurt to do some basic crunches and push ups each day. You'll feel better about yourself internally and look better externally. If you have any sports such as running, bicycling, hiking, if you exploit them in a positive manner, you may find yourself unintentionally exercising and accidentally gaining a more attractive physique. Either way, you'll do fine, unless you are extremely overweight, in which case, this article may not help you.
Be funny at times, but also sincere too. If you look deep into her eyes at some point, even if it's just to thank her for something, she'll be won over; this has got to be done sparingly! No girl wants a boy who is overly serious and always sincere, so just make sure it's tactfully implemented. Give her a taste and then immediately switch to something funny or irrelevant. If you linger in her eyes too long, she'll get seriously creeped out!
Realize that personality is the basis of all attraction. Be nice, polite, sensitive and aware of our female needs, and don't be afraid to use those brains of yours! Every girl under the sun believes the intelligent male is as sexy as it gets, especially when it is accompanied by wit, style and personality. It is a serious turn on when a guy shows a little intellect every once in a while.
Don't be jealous. You're not committed, so just smile and go with the flow. If she introduces a guy friend to you, smile, say hello and immediately shake his hand and make small talk. If you're confident in yourself, she will more likely be confident in you, too. Plus, she may ask the guy friend later what he thinks about you, & you want to make a good impression.
Learn that women love a confident man. They are extremely adept determining your level of confidence reading your non-verbal communication. They will pick-up your non-verbal signals that you wouldn't even have a clue about.
If you are at a bar or a club, and you see a girl you want to talk too, try to be calm, and get eye contact first, you can see if she finds you interesting, and then make your move! Good luck!
See if you like any thing she does. If you do talk to her about that stuff.
Talk about things in your life that relate to hers, and then let her converse with you. Everyone likes to talk about their lives, and what better way to talk about it than with someone else sharing their interests?
Recognize that often the more you like her, the harder it will be for you to try to develop a relationship. Don't let this happen - have self confidence because if you don't go after it, someone else will.
Play with kids. Girls notice this right away, especially if you are enjoying the kids & they are enjoying you. Guys that play with kids appear tender & compassionate. But don't be Mister Touchy Feely GUY! If you are a kid or teen, do it with pets or babies. Maybe a babysitting date would be good? (BUT ONLY IF YOU ARE RESPONSIBLE ENOUGH FOR IT! If you aren't, don't sweat it. You are a great guy with a lot to offer.
IF YOU ARE INTERESTED, learn a musical instrument, like a guitar, saxophone, or something else unique. It sets you apart from other guys. Guys who are musically inclined are a definite turn-on. Girls see them as sexy & passionate. If you don't play an instrument, but your girl does, really listen to her music. Most musicians feel their music is a part of who they are. Never make fun. (Remember: It is easy to lose interest in a instrument, so make sure you really want to do it. Not just for this girl.)
Be a gentleman. Some girls may be uncomfortable with this, as they aren't used to receiving any kind of special treatment, but most girls will appreciate it. A guy who opens doors for girls, holds an umbrella for them when it's raining, talks to girls with respect & not like they're "one of the guys", etc. will definitely catch a girl's eye. Keep in mind that the kind of girls that really love it when guys act like gentlemen probably will not make the first move, even if they really like you.
You only live once. Take a risk. If she says no, keep on rolling! Its only 1 girl, there are other fish in the sea.

[edit] Tips

Don't ever try too hard, she'll lose interest fast. Make sure she sees you laughing with your guy friends occasionally. When you make time for her, don't rub it in, but certainly on a here and there basis it may be in your favor to let her know you made the time available, it certainly wasn't out of desperation or lack of things or women to do. She needs to know you have an independent life outside of her.
Make friends with her friends. They may just tell her you're a great guy. Don't get involved with any of them, though.
Make the first move! Courage is attractive.
Bring up the gossip from other people, it gets her excited.
Profanity is not the answer to trying to look cool or tough.
Swearing isn't great you're not going to win any hands by being a potty mouth.
Smile. It's charming.
Stand up for her. If you see someone bullying her or just being a jerk to her, go up to them (in front of her) and say, "Hey, man, knock it off, okay?" Don't act all tough guy or anything. Just be confident.
If you happen to embaress yourself just laugh at it. Girls love someone who can just like that. But never laugh at someone else laugh with them
Sometimes the ones that are not so popular are the best ones for you.
Make sure that the object of your affection doesn't have a boyfriend. Be subtle in determining whether or not she has a boyfriend [for example, look for a ring]. It's generally not recommended to ask her directly, as it tends to come off as brash and self-centered.
Wink at her
Take out a pen and write your number on her hand, then say "call me" in a deep (not too deep or fake) voice. Do it and look in her eyes smile and walk away, you have her.
Compliments makes a girl feel good about herself and more willing to open up to you!
Joke with her. Doing that makes her sure that you are confident and have a sense of humor.
Try to be her friend first.
Don't just go up to her and ask her out first. Start a conversation and go from there.
Try not to embarrass yourself.
When conversing with her as well as other females, position yourself closer to her. No matter who the other person is, do it. It shows that you're into her. It's subtle body language.
Don't use cheesy pick-up lines such as "Do you have a mirror in your pocket?..." This is pointless and will most likely ruin it for you.
Randomly walk up to her and give her a compliment and walk away. This will make her happy and become interested in you.
Give her a hug from behind and hold her for a few seconds. But don't be too clingy. Its really annoying when a guy wont leave a girl alone.
If you make fun of her she probably will not become interested, and might even become angry with you.
If she's shy don't automatically dismiss her and talk to someone else, you were the one who was interested in her first

[edit] Warnings

Don't talk too much (or brag) around her. Let her do some of the talking.
Do not ask questions like "are you okay? What are you thinking, blah blah" or make statements such as "I'm sorry" for no apparent reason repetitively. This is an absolute no. Some girls might find it cute, though.
When you speak, be concise, don't ramble. Your words will lose value. If you have a great story, tell it, then shut up and listen. ;) Remember that old overused saying? "You have two ears, and only one mouth." It's good advice. Silence is better than babbling.
Don't look/speak about other girls when you're with her. It will make her jealous.
Don't try to be a "manly man." Women often like men more when they're willing to cry if they're sad, and willing to admit their mistakes. You cry too often though and you'll be fed to the dogs or black marked as "broho," meaning a "male ho." or JUST FRIEND
Don't talk about previous relationships! Even if she does it, just smile it off, passively change the subject, or crack a joke that will help her reflect. But don't talk about your ex's.
If she already has a boyfriend, you're messing with fire, however, if she's having problems, you may be the dog to fix them when she figures out how great of a guy you are! If she's in a good relationship or has no interest, respect the fact and move on.
Don't flirt with other girls in her face if they flirt with you, passively let it go and don't react. This will both raise your curb appeal as well as make you seem decent and more desirable. Make sure that you don't make her feel lame when she is with you.
Don't be a full of yourself guy. A little bit of obvious insecurity will probably make her think you are less secure.
Let the girl feel like they can trust you so it will work well on both sides.
If you are at a bar or a club, try not do drink too much. Some girls see that as a turn-off!
While it is good to be her friend first, do not leave it like that for long, as you can easily become "just a friend."