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.
Freelancer – refer people to developers, designers, and other freelancers you’ve worked with and make 100% of Freelancer’s project commission for the first 90 days. I get a lot of people requesting WordPress speed optimization services… so I refer them to my developers with a freelancer affiliate link and make $125/month in passive income. You can’t use affiliate links to link to specific freelancer profiles, so I direct people to the homepage via affiliate link and give people my developer’s usernames.
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.