I think, though, while you do need to know at what point you've scored the number, and know how to escalate via phone text game, it's important to know how to get that number smoothly so that it feels natural and doesn't become an awkward moment.
Definitely searching for a piece of paper and something to write with is outdated. Business cards are too... well, businessish in my opinion, and they just get lost. You gotta do it in a smooth way so that it flows with the interaction and there's not a whole lot of time for her to hesitate.
When you manage to get each other's numbers in your phone, you can, at the very least, text her with the classic, "Which Diana is this?"
http://dallaspua.com/forum/viewtopic.php?p=3637&highlight=#3637