Lawgeek (who's just quit his job to become a university prof) posts a roundup of students' how-to-cheat YouTube videos. The best one is definitely the guy who scans the label off a Coke bottle, replaces the nutritional information with cheaty stuff, prints it, and glues it around a bottle (presumes that your teacher lets you bring Coke into class — I suppose this works best in schools where Coke has struck a deal requiring their products to be available at all times and in all places.)
When I was a kid, we were obsessed with figuring out methods for cheating — far more so than with actual cheating itself. We used binary encoding to sneak in long lists of numbers, stitching them up the outer seams of our jeans or cuffs — a stitch for 1, no stitch for 0 — that we could read by fingertip. After we learned the resistor color-coding scheme, we started to shave pencils and then decorate them with colored bands that actually contained the same lists of numbers. We tried — and failed — to produce a decent tapping code for interactive cheating, though this is certainly possible. One exciting failure was a light-based semaphore wherein the conspirators would flash reflected discs of light up on the wall over the teacher's head using our watch-faces.
The kids in these videos are awfully sanguine about their teachers' YouTube cluelessness. I'm relatively certain that the adorable little English moppet pictured here has never actually succeeded in using his cheat, as it relies on your teachers allowing you to keep playing cards on your desk during the exam. This is surely a purely theoretical cheat.