If you've ever seen Scott Delea of DigitalGrit speak about successful search engine marketing, you've probably heard of the Taguchi Method. It's a really cool method of multivariant testing - initially used in manufacturing - that's been applied to direct marketing. As Scott put it, "he created a test matrix that dramatically accelerated his understanding of which factors contributed to a better or worse product."
I found this great article on Taguchi on PBS.org (thank you, Google) that I really had to share with anyone who ever asked Scott, "What the heck's a Taguchi Method?"
Here it is. (I really wanted to quote it in this post, but it seems to be protected from copying.)