Well always act like you have a better place to go and better things to do, and when you brake out of the set, you could rip a piece of paper out and put your number on it than hand it to her and let her put her number down. Thanks lot David D for this.
What i like to do, and this was inspired by Style, is to give her a piece of paper and pen, and ask her to draw herself (assuming you played good game she will) than ask her to draw a phone in the picture and put her number on it (something silly like this, no one does it, its fun and shows your different).
Or you could do the email close, hand her a piece of paper say " I have to go you've keept me long enough

, put your email on there well talk about (whatever your game was, female things are usually best, clothes, shoes, going shopping with her so on, or you could do the cube game and go half way, than ask number to resume it later [Time bridge] ). And as she puts her email on the piece of paper say, "Oh while your at it, put your number there too".
Hope it helps.
-Impact