To keep track of what key opens which lock, simply trace the outline of each key on a 3-by-5 index card. Write down what lock it fits -- whether office, home, a gym locker, a bicycle padlock or a storage trunk, or maybe even grandma's house. Then file the cards away in a safe place for future reference. You can even tape the keys you're not using to the correct card and store them until you need them.
(Taken from the Everyday Cheapskate newsletter)