How to Forget About a Girl You Love Who Has a Boyfriend

Start liking someone else quick. In this position of depression, you are very vulnerable to emotional attacks. Liking someone else will distract you from that old girl.
Don't listen to any slow or love songs... you will grow desperate.
If you are friends with her, make it clear to yourself that friendship is all she wants. She has her boyfriend because she likes him!
Be sensitive to her, but not overly. You'll just be digging a deeper hole for yourself.
There are other women, who will accept you! Go for it.
Go out and have some fun. Don't just stay at home thinking about what she's doing with the bf.
Don't cry over a woman who doesn't like you.
Do not let yourself think that you are not attractive, or that you will never find someone better. There is always someone better.
If she has a MySpace, Facebook, etc, avoid her page. Seeing her and her boyfriend there will just hurt.
Don't have her within your top 5/10/etc, unless you are practically best friends. She'll just be suspicious.
Try not to look at her.
If you start to think about her, then think about someone else. Someone hotter.
Don't be calling her a lot. That's just not cool.
Unless you have a legit reason, just don't call/text her.
Don't call her babe, baby, etc.
Don't look at her as the most important person in your life.
Say no to her once in awhile.
If you are a student: Forget her because it could effect your mental health, which will directly affect your studies. After all, your degree is more important.