Oh! I’ll add this – if your individual site or blog is applicable, All Posters has an outstanding affiliate program. I get healthy payments from them each month. They have more than just posters – they have art prints, wall murals, t-shirts, wall decals, etc. A lot more people order these online than you’d imagine. I wouldn’t give them up for anything.
Research individual companies in your desired niche: If possible, it’s always better to become an affiliate directly with a company (if they have an internal affiliate program), as no one else will be dipping into your commission rate. This is the preferred route for most of the prominent affiliate marketers, including Pat Flynn. Unfortunately, it’s also the most work, as you’ll have to do the research yourself to see who offers programs (they’re usually listed in the website footer).