Requirements:
1. Small pad of paper
2. 3 sheets removed from pad labeled "1", "2" and "3".
3. A pen
4. A trifold wallet.

Before you go out, slip the 3 pieces of paper in and around your wallet in any order (just make sure you remember it!) and stuff your wallet with the papers in your front pocket. Then, when your talking say, "Hey how about a magic trick" or what ever. Take a piece of paper out of the pad and pretend to write a number on it. Then stick it in your pocket, but crumple it up when you hand is in fully submerged so you dont confuse it with the others. Tell her to pick a number between 1-3 and then all you have to do is remember which paper is where.

Note: you can do this with more numbers, but just be sure to memorize their placement.