I recently asked Michel Fortin for some advice on testing sales pages. His advice was great, as always:

When I think about elements to test and that convert the best, the two most important ones are the headline and the offer.

But other than that, I think of the AIDA formula. That is, anything that helps to draw attention, create interest, increase desire and drive action are elements I want to test.

For example, when it comes to attention, think of the things that help stop people from scanning/surfing, and forces them to start reading. From headlines to pictures, from pop-ups to video, from different colored backgrounds to different colored headlines.