I just join some affiliates programs periodically and see which ones work best for my blog audience and my own profit of course Some affiliates programs are not worth it at all, so you must be picky and analyze your results monthly.
If anyone's interested there's a new good program from a new SEO tool called WebMeUp (info is here http://webmeup.com/partners/affiliate-program.html).
I don't know why but I manage to generate the majority of my income through this program. Maybe it's because the tool is just great and is perfect for SEO.