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.

Interesting. What sort of period of time? Years seems like the best way to cover a decent range of the market. Got any links?

You will beat the "average" value of the market.

EX: While buying 100$ in stock at 10$/share and 100$ in stock at 20$/share means the average price was 15$ you have 15 shares for 200$ which is 13.33$ / share.