I've got a couple more tips to choose the best product:
6. It offers lots of tools to use, like banners, sales copy, an auto responder series.
When the author has taken the time to give me all the materials I need, it makes the decision much easier. I especially like when I get extra bonuses. I know the other affiliates can use them, but the customers don't usually know that
7. It's a recurring program. I love membership sites that give me a percentage each month one of my referrals pays. Since it's usually set up automatically, the customer gets billed every month and I get paid every month. Woohoo!
If they don't have a recurring program, it helps that it's set up so the customers have a reason to come back. Like offering free upgrades or email support.