One great way to get ideas for related products to promote is to look at the stats/reports that Amazon gives you to see which products readers are buying. After a while you’ll start to notice that they’re not only buying the products you directly promote but other products as well. Some will be completely irrelevant to your niche – but many times trends will emerge that could signal other products that it might be worth promoting.
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.