You're going to be doing product reviews and recommendations, so pick a topic that you enjoy and about which you can demonstrate some expertise. Choose a narrow enough niche to be distinctive—for example, bands from your city, left-handed guitarists, music for a certain kind of dancing, authors of a certain religion, books about business, or arts and crafts resources. If you can't stay passionate about the topic, that will show.
Perhaps most importantly, though, successful affiliate marketing on Amazon is built on the same foundational principle that all affiliate marketing, and all online marketing and business, is based: trust. In the end, Amazon is another tool that helps you help your audience and build their trust further, by promoting products that will help them achieve their goals.
You should also make sure you aren't competing with your own affiliates for eyeballs. Any marketing channels you're using, such as search engines, content sites or e-mail lists, should be off limits to your affiliates. Put marketing restrictions into your affiliate agreement and notify partners immediately. It's your program--you set the rules. Or, if you prefer, you can let your affiliates run the majority of your internet marketing.