What kind of commission do they offer? One-time commissions or recurring commissions? For example, many programs pay you one time for sending a customer. On the other hand, some programs like membership sites or SaaS (software as a service) programs will pay you a commission as long as the person you referred is a paying customer. Recurring commissions are great when you can find them!
Know when (and when not) to use Viglinks and Skimlinks. If you applied to an affiliate program but were denied, you might be able to still be an affiliate for that advertiser through a secondary affiliate program like VigLink or Skimlinks. Basically, they themselves are affiliates and will split their affiliate commission with you if you put their affiliate link in your content for an advertiser. Obviously, the commission rate is lower for you in this case, so if you ever are accepted into the advertiser’s affiliate program directly, immediately switch from using VigLink / Skimlinks affiliate links to your own.
Know what sets each program apart. Affiliate programs are everywhere online, and it can be hard to tell them apart. Each program distinguishes itself by the products it sells. Some affiliate programs only require you to post links; others will ask you to pay money to learn about how to sell the product you are selling. As the latter type of affiliate, you can make commissions by getting others to pay—by bringing other marketers into the fold. Be wary, however, of any program that asks you to pay.
Amazon's commission rate is pennies from each dollar of merchandise that you sell. What many people don't know is that there are affiliate opportunities out there that pay commission rates that are much more favorable, from $20-$100 (and even more). Browse affiliate networks like Commission Junction, Share-a-Sale, Rakuten and Ejunkie for opportunities and their general commission rate.