Rubber Band Method

This is best for formula, etc. and is done quite simply.

1. Take a wide rubber band, stretch it out really widely and place it around books to keep it from getting small again.
2. Write your notes/cheats on the stretched out rubber with a black ballpoint pen, making sure to write your letters as close as possible, while it's stretched out.
3. Let it return to its original size, and the notes will just look like black boxes, not cheats.
4. Wear the rubber band as a bracelet to your test, and when you need the answers, just stretch the rubber band out, and when done, let it snap back.