One thing that I look for in an affiliate program is a good transparent analytics system. I want to be able to see the clicks and transactions, so that I can track my stats. Otherwise, I get a little nervous about the legitimacy of the program. Other things to look for is to see how transparent they are when answering questions and how the responsiveness of the affiliate manager.
Although this article has been written in 2007, now in Feb 2014, I still find it full of great advice and tips. What I learned from this article is that when writing a review I should focus on how this product helped me achieve X instead of simply repeating and enforcing the features and benefits of the product that can also be found on the sales page.