Participate in Affiliate Programs

In an affiliate program, you agree to run an ad, usually a small banner ad, on your site that links back to the sponsor’s site. The ad will contain numbers or words in the link that will enable the sponsor’s site to recognize anyone that clicks through the ad as a visitor referred by your site.
If that visitor buys anything from that site once they click through, you receive a commission off that sale. Finding affiliate programs are easy to do. Look for sites that offer products or services that compliment yours, but don’t compete. Check their site for affiliate programs. Don’t sign
up for just any affiliate program, however. Again, you want to make sure the site already gets plenty of traffic, that they have a good reputation, and whatever product or service they’re offering goes with your site.
Google AdSense is very discriminating about having other ads appear on the same page as theirs. Affiliate ads which contain graphics and not text are usually okay, but be sure to read your Google TOS thoroughly so that you are not in violation.