[edit] Steps

Avoid becoming obsessed. No one likes a stalker. This does not mean not to hang around with her or go places with her a lot. It just means that if it's quite obvious she doesn't want to hang around you, or if she is doing something private (such as a get-together with close friends), then give her some space.
Do not hit on people continuously, especially not in front of your girl. She could get the feeling that you don't like her. If you hit on her, make sure you don't do it all the time.
Make sure you smell good -- hygiene is important. No one is going to want to hang around with someone who smells bad.
Look good. She won't like it if you wear the same thing all the time. Just try to switch it up. You don't constantly have to change outfits. You might even want to joke about how to always wear the same clothes.
Be nice to her friends. Don't give the impression you are interested in them beyond friendship. This is a big one. If you become friends with her friends, she will love you for it.
Don't try and force anything on her. If you come out straight away and tell her you are interested in her in "that" way, she may become uncomfortable around you. Keep in mind, some people would prefer you to tell them straight up.
Don't come on too strong at first. Be casual and cool. You'll know if she wants you to come on stronger. If she is coming on stronger than you are, that generally means you can step it up.
Do not say that you are going to kill yourself if she gets mad at you, that may make her think that you are slightly obsessive, and have nothing better to live for than her.

[edit] Tips

Don't become some complete other person for her. The act will be hard to keep up.

[edit] Warnings

If she knows that you are interested, she may try and use you. Be careful of people who have been known to do that.
If she's not interested, she's not interested. You can't change that.