Select the right program

Even though you are free to offer anything you like to your site's visitors, it's best that you select an affiliate program that matches your web site's topic. If your site's topic is about automobiles, don't sign up with an affiliate program that offers/sells computer software, instead, sign up with a program that offers ...let's say...car accessories.

Don't forget about commissions

The main reason why people/web sites, sign up with affiliate programs, is to make money. Therefore, when selecting an affiliate program, make sure that such program pays high commissions.

Commissions vary by program. Some programs like Amazon.com pay up to 15% commission on sales generated from clickthroughs that come from your site. Other programs will pay you between 5% to 10% commission on sales.

Finally, don't forget to reevaluate your affiliate program every few months to see if it still suits your needs.