Locate online communities relevant to the affiliate you are promoting. This might include forums, discussion boards, chat rooms, and blogs. Post often on the forum, and build a good reputation among the community members. Once people trust your judgment, you can start to include affiliate links in your posts. Do not simply spam the forums with product codes. You should sprinkle the affiliate links within genuinely useful content.
Watch them. How are other content creators in your niche utilizing advertising? What types of ads do they use? What are they promoting? What do they talk about repeatedly (if you keep seeing the same affiliate product show up again and again, there’s a good chance they’re making good money from it)? If you see a product or service they talk about that jogs your memory and you can ethically promote it too, find affiliate information by the methods described above.
The problem with affiliate marketing, like many other home business options, are the so-called gurus and get-rich-quick programs that suggest affiliate marketing can be done fast and with little effort. Odds are you've read claims of affiliate marketing programs that say you can make hundreds of thousands of dollars a month doing almost nothing ("Three clicks to rich!"). Or, they suggest you can set up your affiliate site, and then forget it, except to check your bank deposits.