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.
Selling T-shirts and other apparel is a pretty saturated market but the tools to do it are easy to use, which makes it very quick and cheap to get going. If you’re creative enough, or tap into the zeitgeist properly, you can also have a runaway hit. Even if your first 19 T-shirts don’t sell more than a few copies, your 20th might make up for everything.