Step 1: Magnet Mania!
Look on your family fridge for cool magnets to use. Souvenir shops also have some good magnets. Your local officemax has some nice ones too.
Making your own magnets, however, is much more fun and rewarding. Buy some magnet sheets with the sticky peel-off back, available at OfficeMax and at local craft stores. Pretty cheap. Next, find anything small enough to fit in your hand that you would want in your locker! Some good suggestions are:
Birthday cake toppers
Game pieces (checkers, chess, etc.)
Stickers (with backing still on)
Make sure to cut out just enough magnet for the figure.
Also, you could magnet a small notebook and a pen/pencil to your locker for easy access.