Buy low, sell high!
Sounds simple, doesn't it?
But, there's more to it than meets the eye. If a stock goes down while the rest of the market goes up, or the rest of it's type goes up, then it just may be that that stock just sucks [not a good time to buy].
However, if the entire market goes down, and the stock that interests you goes down with it, it may be a good time to buy. Just make sure it's a stock [or part of a group] that leads during recovery.