If you get the green light (steady eye contact, smiles, body contsct is even better - be it just a hand on yours or on your shoulder) guage the right time when you're alone together (she will instigate this, usually). in the midst of normal conversation say "I'm gonna fuck you".

Works every time.

PS. Don't say "I WANNA fuck you" - errm, get in the queue, cuz most of the guys do. It has to be "I'm GUNNA fuck you.

Makes all the difference and I never yet met a girl \I said tht to that I didn't fuck.