How to get her
Beat her up.

Competition
Ranma and Mousse. First, you'll have to beat up both of THEM. That will probably be that with Ranma, but Mousse is just a teensy bit psycho and he worships Shampoo, so you'll have to beat him up every couple of days.

She likes you if...
She'll either run over you with her bike or show you her boobies. She won't try to put any spells on you or drug you unless you play hard to get.