As a follow-up post to Optimizing Your Web Site vs. Optimizing Your Search Rank, here's a 25-minute video on how to use Google Website Optimizer:
Of course, you've got to start by measuring your site activity with (free) Google Analytics. If you haven't done that yet, here's How to Sign Up for Google Analytics in Less Than 5 Minutes (Quicktime movie).
Do you have questions about all this Web site optimization stuff? Leave your questions in the comments!