FriendFinder is an adult-friendly network of dating websites that has a terrific affiliate marketing program, both in terms of customer service and commission rates. Because they rely heavily on affiliates to recruit new members, they treat their affiliates like true business partners. They have a solid reputation for payment and security, and have frequent special offers. Checking into your affiliate account at FriendFinder is always a fun experience, and often a profitable one.
The topic you choose must have enough depth that you can create a lot of content for it. This is important for building an authoritative site, for search engine optimization, and most importantly, for the end user. If you don't have enough content about a topic, you're not going to be taken very seriously as an authority on the topic and it's unlikely you can convince someone to make a purchase from you.