There are a lot of companies out there offering affiliate opportunities, and personally I feel it is too much just to fill up your blog with all, or even most of them. I think it's important to be selective and choose those that you feel would be most suited to your readership. Most companies also give you the option to select products in line with the content of your site.
I'm still at the stage of trying out these companies at my various blogs, to see which ones I should stick with. I can imagine myself at some point in the future, eventually settling down with the most productive.
Some recommendations to become an affiliate are the following -