These two lines worked on me:

"You're really cool, we should hang out sometime. Can I get your number?"

"Okay, when is a good time for me to call you?"

Then call her on the day she says.

(This may be crap advice, because unlike some girls I'm just sort of friendly and will go on at least one date with someone even if I don't know if I like them. And I find punctuality, straightforwardness, and kindness refreshing. No need for silly "I'm trying to be seductive and keep the mystery" boy-games with me, because I suck at playing girl-games back. But maybe I'm weird.)