The affiliate program should be content related. When deciding on which affiliate program(s) to place on your site, consider the web site's topic. What is the site about? Who is your target audience? If your site is about baseball, you might want to consider placing sports related affiliates on the site that offer sporting goods, supplements and things related to the content of the web site.

Review potential programs. 
Before deciding on a program, you should ask yourself, "Do the products and/or services offered have the potential to generate revenues for my site?" You should take a look at performance ratings, feedback and historical data for the program. Most affiliate programs offer some sort of statistical data that details average earnings per thousand visitors. 

As you run the affiliate's advertisement on your web site, closely monitor performance. Again, most affiliate programs offer tracking and reporting tools than can be very helpful in measuring profitability. A major plus for affiliate marketing programs is that most are free to sign-up on and allow you to simply test a program and see what kind of results you get. Remember targeted marketing, "Who is my audience?"

Convert traffic into earnings.
Getting traffic to your web site is top priority, but is no guarantee for any kind of return. If a web site lacks content and is filled with flashy banners, the site will fail to generate any return. Content is king, and the content provided on a web site can make or break the site. Again, consider your target audience and build your affiliate marketing programs around this group.