There are two broad categories under services: earn referral fees on a click-through basis, or earn fees for recruiting a visitor to an e-mail list or other service. A simple option is to use search boxes from some of the major search engines. GoTo (www.goto.com), Lycos (www.lycos.com) and Excite (www.excite.com) will pay about US2-3c each time someone uses the box on your site. These are very successful at the end of articles where you can prompt people to look for additional information by suggesting phrases that will help their search.
The high-paying end of affiliate services is encouraging visitors to sign up to e-mail programs, online sweepstakes or dating services. The fees vary from US20c to $US2 but you will need to be very selective about which programs to promote - plus, you must ensure that they are not competing with content on your site.
It can take time to locate suitable affiliate programs and one of the quickest options is to search an affiliate recruiting site:
Be Free (www.befree.com) is one of the best - it also tracks sales and provides HTML generators for the links.
Refer-It (www.refer-it.com) has close to 1000 affiliate programs, including many banner ad programs and a rating system.
Associate-It (www.associate-it.com) is another large directory similar to Refer-It.