My friend does this amazingly well and it works excellently: Accuse the girl of doing something disgusting, or ask a question to that effect.
He's been using this one lately with much success, inspired by the book I Hope They Serve Beer in Hell if anyone's read it:
"So let me ask you something because it's really been bothering me lately. Are you the kind of girl who would suck a **** and then go on a date with a different guy the same night? Because I read about that happening and find it really disturbing to think some girl may have done that to me in my lifetime."
And of course she'll say NO!! and of course you accuse her of being a liar. From there use your imagination, at least you're talking about sex now.
Don't be too accusatory though, of course always keep it lighthearted and fun and smiling and don't REALLY accuse her of doing this, even though that's exactly what you're doing.
P.S. on the off-chance she has to think about it or says yes, sincerely, you've found yourself a winner sir, escalate and stick it in her posthaste.