http://en.wikipedia.org/wiki/Paul_the_Octopus
There aren't many possibilities, if enough people try, there will be a lot that "predict" it right many times in a row
The successful ones can also influence others to sell when they say it goes down and such making the prediction selffullfilling (it's not clever to follow that though, because the only one to make certain money of that are the ones that do the prediction, because they will be the first to buy/sell)