There was an amazingly beautiful girl?

with long black hair, great face, was wearing short shorts, was really nice and fun to talk to, had a near photographic memory, and I didn't ask for her number... :(. What's a good way to ask for a girls number? I've dated women but they tend to be people in my classes, work with me, or know friends I know. How do I ask a woman for her number after meeting her for just a day?