What size hands do you have? Small, Medium or Large?
What grip style do you have? Claw, finger tip or palm? Which will effect the shape and size of the mouse that you want.
And do you want a heavy or light mouse?
And how much are you willing to spend?
The main other point to look out for in a mouse right now is which type of sensor you want. Optical or laser. I personally find optical mice much more precise over laser.
Also don't get taken in by the DPI hype that the manufacturers are advertising. No one needs 12000 plus DPI. About 2000 to 5000 should be more than enough DPI for your needs.