I don't disagree with evansteve's "ask for the number after a minute or so" of talking with her. She'll know if she likes you well within that first minute and I'm certainly not one to play games that way. If you want to see me again, give me your number. PERIOD. Evansteve, I don't wanna confuse you, but that's how I did it all the time.
You don't necessarily have to ask specifically for their digits, but can say "We can go out sometime" or "We can do that" (whatever you're talking about at the time) and that would of course entail getting their contact info. I don't ask all the time, I TELL them "We'll do that" or "I'd like to do that with you, what do you think?" even after one minute, if they like you they'll say "YES" and give you their number.