A Makaque monkey over on Twitter told me about a great article at Get Elastic on how a retailer tested two versions of their email to see what worked better:

Which version do you think got more clicks and 15% more sales?

Want to run tests with your own emails? It’s free in MailChimp, and it’s unbelievably easy and automatic.  Even a little fun. And people who use it get better results than people who don’t. It’s called A/B testing, and here’s how it works.