Here is an excellent video from Ryan Carson on how to quickly set up and start using an A/B Testing plugin for WordPress.
How to do A/B Testing with WordPress from Carsonified on Vimeo.
The importance of A/B testing really can’t be overstated enough, it is one of those things that you really don’t think has the type of effects that other people talk about until you try it yourself. And with tools like this making it stupidly easy to do this kind of testing there is really no excuse.