slap her around the face with your penis.
ok, maybe don't do that, but seriously dude I'd say you shouldn't even try..once you've been placed in the "friends" category its mighty difficult to get out again.
sorry to be so negative but what you're asking to do isn't an easy task and its probably a much better idea to just next her..and save yourself a LOT of time and effort.
however, if you really can't do that and are desperate to try something then:
start flirting with her (making fun of her, teasing her etc)
start touching her more (not sexually - especially not at first)
don't listen to her problems with other guys/be her shoulder to cry on..etc
I'm sure some other guys on here can give you a few more pointers
good luck dude, you will need it.