Nick...

The basic theory behind straight Dollar-Cost-Averaging is that if you invest a set amount of funds on a regular (say monthly) basis, that you will buy more shares when the price is down and less when the price is up thereby achieving an average share cost that is generally better then if you had tried to time the market. This article explains it better:

http://www.americancentury.com/workshop/articles/dollar_cost_averaging.jsp

Hope this helps,
Bill