This has become a popular business model for online entrepreneurs over the past several years, and will probably just continue to grow in popularity. The best thing about selling online courses is that once you do the up-front work in creating the course and setting up your marketing strategy, you can get paid over and over again for work you do once.
Let no one tell you that email marketing is dead. An email list is crucial for every affiliate marketer. You can start building up your email list with a lead magnet (like the information products mentioned previously) or even just by encouraging your audience to sign up for your updates. You can then push your content to this audience via email and also direct them to your affiliate offers. Don't be sleazy about the sales, but if you build up enough trust with your email audience; when the time comes, they will not mind purchasing a product from you.
Research individual companies in your desired niche: If possible, it’s always better to become an affiliate directly with a company (if they have an internal affiliate program), as no one else will be dipping into your commission rate. This is the preferred route for most of the prominent affiliate marketers, including Pat Flynn. Unfortunately, it’s also the most work, as you’ll have to do the research yourself to see who offers programs (they’re usually listed in the website footer).