Long story short: you can't predict the market. If you buy all your stock on one day, and in hindsight it turns out to have been at a peak, we'll you're screwed aren't you. If you buy your stock in periodic roughly equal sized increments over a period of time, the average cost of the stock will correspond to the stock's average over the long term. (i.e. sometimes you get it "cheap" when the market is beaten down, sometimes you get it "expensive", but it all comes out in the wash.)

This is a common strategy for long-term investors.