It turns out you don't need to be a savant like Dustin Hoffman's character in Rain Man, or an MIT mathematics prodigy in order to count cards. There's a relatively simple approach that can give you a slight advantage over the dealer when playing Blackjack, and this site can help you master it.
You've probably assumed that since you can barely remember your telephone number, memorising every card that's been dealt from a deck is an all but impossible task. But that's not how this approach to card counting works. Based on the face value of each card you see in play, you simply keep a +1 or -1 tally in your head. As a game progresses, that number, whether it ends up positive or negative, gives you a good idea if the remaining cards will work in your favour, or the dealer's.
If it still sounds complicated, you should really just head on over to this site and try it out. It explains in more detail how and why this technique works, and lets you test your card counting skills against a computer dealer.
At set intervals the site will quiz you on the running count, and over time you can adjust how quickly the interactive trainer deals cards, since the dealer in a real casino isn't going to stop and wait for you to check your maths in your head.
Don't expect to walk into a casino and clean up at every Blackjack table after mastering this technique, though. There is still an element of randomness to the game, this just helps give you a slight advantage if you know how to play the game, and bet properly.