Well, try to imagine 5 things about her that would make her unattractive to you. They can be physical or mental, or whatever. Imagine her as 50 lbs heavier, or really boring, or someone you wouldn't trust, etc.
Also imagine her in some position of vulnerability. Imagine her giving up her rights w/o a fight when being pulled over by the cops, imagine her being made fun of by some people because she said/did something stupid and her just sulking since she can't handle it, imagine her afraid to speak up in class for fear of looking stupid, etc. If you do those things, the other person isn't nearly as intimidating and you don't fear rejection as much.
I have no real problems approaching, too bad I suck at the actual pickup part