A few things to consider before signing up with an affiliate program:

• Does the affiliate program offer real-time stats? A quality affiliate program will give you access to detailed statistics on sales, traffic and commissions. Top quality programs will offer other features like email notifications--tools that can help you configure the best possible setup for maximum profits.
  
  
• Is the money you can earn with the program worth your while? Choose your affiliate program carefully! You may end up spending weeks or even months setting up an affiliate program partnership only to find out months later that the earnings are pitiful. Do at least some research; find out what others are making with a program. Search the Internet for forums that have discussions about the affiliate program you're interested in. A little research can go a long way.
  
  
• What kind of support is available? Quality programs will provide access to helpful and efficient support. Find out if the program you are interested in has a robust online help section of their website. In my own experience, I've found that testing a program's email support is a very good way to gauge the quality of a program. Before signing up, send an email query to their support email address (or use their support email form) and see how long it takes for them to respond; also, pay attention to the quality of the response. If a program's support system is lacking, it's a pretty good bet that the overall quality of the affiliate program is no good as well.
  
  
• Will you be competing with the company for traffic? This is a common problem that you should look out for. After spending much time setting up a website and going to great lengths to optimize your site's content for specific keywords, you may find that the company you've signed up with is in direct competition with you for those same keywords! Not a good idea to try and compete with the very same company with which you've become an affiliate!