Women, what makes you say "yes" to a date?

What I mean is, out and about in my daily life I feel like I run into a lot of adorable and beautiful women, from the girl that works at Jamba Juice to someone I pass by on the street. Most of the time I'm pretty laid back and calm in striking up a conversation and flirting a bit but when it comes time to ask for a number I clam up.

My question is this: What makes you want to give out your number and see someone in the future? How many times does this happen to you in a day/week?