Never give them yours.. I've only done it once (waitress) and though it did it the best way i could. Waited untill she was away from everyone at the till and no college's near her. Just went direct and said i though she was cute and gave her a card with my number on it. Never rang. I'm guessing it's always best for you to get their number as you have a little more control in the interaction for their on in. :-) My lesson learned