If you have a propensity for writing and you can slay with your prose, consider writing an ebook. While the market has certainly become saturated as of late, books that help teach people about a technical topic still sell extremely well. This is a great source of passive income but does require a large amount of effort at the outset before any money is generated.
Good comment Jason, at some time in the future Amazon may decide they have so much market share they don't need affiliates anyway. I mean, if you're just sending them people who are already Amazon customers there's not so much benefit there for them. Or they may decide to only work with select HIGH QUALITY affiliates and the average "affiliate site" owner will not be chosen.
Regardless if you need to earn some fast cash or we're just talking about making money in the grand scheme of things, there's an important psychology that needs to be mentioned before getting into the strategies. If you study Freud's model of the mind, you'll discover the Psychic Apparatus. It's the three-part construct in your mind that controls all of your behavior.