Crushes are triggered by your mind. Believe it or not, your conscious mind. That means that if you actively keep searching for her good points, imaginary situations, or the ways in her attractiveness, then you will feed the "incontrollable" crush, ie. stimulate your brain to produce endorphines and what not that will make you lose your rational mind. Basically, you are practically inducing yourself into a hypnosis to produce chemicals that produce the feelings of stress. It gets to a point where it affects your conscious mind, and your ability to think, and take you into depression.
Make a conscious effort to avoid such thoughts, and as >>2 already pointed out, try to regain homeostasis by having a healthy diet and lifestyle that balances out all the nasty chemicals in your body and add good endorphines to your daily menu (excercise is a good way to secrete endorphines. Sex is an even better one. ;P) Trivia: Did you know that your brain secretes a painkiller stronger and more effective than heroin? >>2 The cocaine simile is not very good tho.