Sign up for a reputable affiliate network: Aside from Amazon, there are dozens of large reputable affiliate networks, such as Share-A-Sale, Clickbank, and Skimlinks, that specialize in connecting you with merchants who are looking for affiliates to sell their products. They charge relatively low commission fees for the privilege of connecting you with merchants, and the merchants on these sites tend to offer much higher commission percentages or set dollar amount payouts.
One of the cool things about Google AdSense is that it's so easy to get set up. If you have a blog or website, you can sign up for a free Google AdSense Account. From there, Google will give you a unique code that you will paste onto your website. Google takes it from there, tracking your page views, traffic, and earnings on your behalf. There is no upkeep or maintenance to get this thing going, which makes it a no-brainer if you have a website already.