Illusion.
It's not the tricks you should be worried about, it's the person doing them.
David Blaine has a very hypnotic voice, Darren Brown psyche's his audience up to the point where they believe almost anything.
Something else you should know about called card forcing.
It's a method of the classic "pick a card, any card" trick that involves something really big and dynamic to happen - like the card to be on the inside of a window or something like that.
Basically the trickster already knows what card you're going to pick, and he's already planted the card somewhere awe inspiring, then he simply forces it.
Forcing a card is just giving you the illusion of freedom, where you'd have the choice to pick any card, but with usually a trick deck, so you'll pick exactly the card they want you too.
Look up 'Penn and Teller - Bullshit' for more details.