Cry out everything you have to cry about! When we get hurt, it's normal (and good) to cry. Don't ever think you're being weak for crying and don't feel embarrassed because of it! It's normal and it's good! When you cry, you let go of part of your anger and hurt so you can feel less heavy. You can lock yourself in a room, if you want to, and put on some sad music...but let yourself feel the pain and cry so you can let it go. The main thing here is: Get rid of the pain! Just let her go!
Get busy! When you're trying to get someone out of your head, you need to put other things inside of it. In other words...get busy! It doesn't matter how, you just need to get distracted. Go to a movie, watch a play, travel. It doesn't matter what are you going to do--the important thing is to find something to do. Find a hobby, find something you enjoy doing, something to keep your mind busy. If your mind is busy, it doesn't have time or space to think about the person you're trying to forget.
Spend some time with your friends. Friends are always great to have in this kind of situation! Friends can make you feel good about yourself and get you distracted very easily. They will certainly make you laugh and make you see that you're way more important than you think! The only warning is: Ask them not to talk about the person you're trying to forget. If they start bringing the topic up in every conversation, you won't be able to forget, and instead of making you laugh, they're going to make you cry. So be honest and ask them not to talk about it!
Avoid the person! Try not to go to places you know you can meet that person. When you're trying to forget someone and you two keep seeing each other, it gets hard to get over it. If you meet him/her somewhere, just be nice and kind, but find an excuse to go away as fast as you can! If you work or study with him/her and you can't avoid seeing him/her, just try not to look and not to talk too much. Just be gentle and keep busy all the time so you won't have excuses to look at him/her or to chat.