Products and services in these evergreen niches are always in demand because people are always looking for solutions to specific problems in their day-to-day lives. You'll want to operate in one or two sub-niches, to begin with, because it is impossible to make meaningful offers in such gigantic broad categories. In the example above, "group fitness for women" is much more targeted than "how to lose weight". It's easy to tackle a sub-niche within a large niche then grow from there.
Another brand who has mastered the art of referral is Uber. Each user is given their own referral program, which gives not only the current user but also the newly-referred customer, a free trip. Because of this Uber take the cake in ride-sharing apps and services. Other similar services have adopted such programs and have also seen a large increase in users.
Perrin has blogged about his site on the AuthorityHacker.com blog, and he's also openly discussed the sale of this site, which was sold at over $200K as the final sales price. The site still does very well in organic search rankings, and the monetization appears to have been diverted from display ads, over to an affiliate program with a pet food company. This site is proof that with the right systems and content in place, profitable affiliate sites are still out there to be created – even by the solo entrepreneur or the person who loves SEO as a side-hustle.
This is a great example of a top notch review site. They start at the homepage notifying that they make affiliate commissions, but provide top end reviews from independent reviewers. This is great to be honest up front. In addition to being transparent, they also take the content is king strategy. I randomly clicked on their “Top Home Projector” post where they reviewed (and linked) to several high end home theater projectors. Keep in mind that these high priced items produce high commissions. That may explain why they spent the time to write a 5,000 word post on it. This site seems to do everything right in being a prime example of an Amazon Affiliate Website. This site now has over 60 staff members working for it.