Check out my recent article on affiliate marketing! Even if you don't plan to BECOME an affiliate, using affiliates to promote your business is a great idea and inexpensive way to promote ANY business - Internet or brick and mortar.
Why be an affiliate? Even if you don't want to make it a full time business, it gives you a strong handle on what the public is looking for, what wording they respond to in ads, and how common thought is progressing.