Teller had a great piece in last March's Smithsonian magazine, explaining the overarching principles behind all magic tricks. It's a great look at the way that our brains can be enticed to fool themselves.
1. Exploit pattern recognition. I magically produce four silver dollars, one at a time, with the back of my hand toward you. Then I allow you to see the palm of my hand empty before a fifth coin appears. As Homo sapiens, you grasp the pattern, and take away the impression that I produced all five coins from a hand whose palm was empty.
2. Make the secret a lot more trouble than the trick seems worth. You will be fooled by a trick if it involves more time, money and practice than you (or any other sane onlooker) would be willing to invest. My partner, Penn, and I once produced 500 live cockroaches from a top hat on the desk of talk-show host David Letterman. To prepare this took weeks. We hired an entomologist who provided slow-moving, camera-friendly cockroaches (the kind from under your stove don't hang around for close-ups) and taught us to pick the bugs up without screaming like preadolescent girls. Then we built a secret compartment out of foam-core (one of the few materials cockroaches can't cling to) and worked out a devious routine for sneaking the compartment into the hat. More trouble than the trick was worth? To you, probably. But not to magicians.
3. It's hard to think critically if you're laughing. We often follow a secret move immediately with a joke. A viewer has only so much attention to give, and if he's laughing, his mind is too busy with the joke to backtrack rationally.
(via Naked Capitalism)