Day trading is a whole different world (plus, you'll note I was careful to talk about "rational" investing behavior). The only clearly rational (i.e., even Bitt would agree it makes sense) form of day trading I've ever heard of is currency arbitrage. You program your computer to watch all the pairwise exchange rates. If you can find a cycle that causes your money to increase when you make a complete loop, then you push as much money through that cycle as fast as you can until the cycle closes.

The only other possibly rational way to make money with day trading is to have hair-triggered reflexes combined with hard core financial modelling. If you detect a security that, for whatever reason, has dropped below the price your model says it should have, then you buy, buy, buy. If it gets above, then sell, sell, sell. Of course, your model might be wrong and you could get deeply screwed.