A two-tier affiliate program allows affiliates to not only earn commissions on their own sales, but to also get a percentage of the commissions (usually much smaller) earned by people they've recruited into the affiliate program (either directly because they knew them or indirectly – meaning someone signed up to be an affiliate by using the first affiliate's link).
Some commentators originally suggested that affiliate links work best in the context of the information contained within the website itself. For instance, if a website contains information pertaining to publishing a website, an affiliate link leading to a merchant's internet service provider (ISP) within that website's content would be appropriate. If a website contains information pertaining to sports, an affiliate link leading to a sporting goods website may work well within the context of the articles and information about sports. The goal, in this case, is to publish quality information on the website and provide context-oriented links to related merchant's websites.
You should also make sure you aren't competing with your own affiliates for eyeballs. Any marketing channels you're using, such as search engines, content sites or e-mail lists, should be off limits to your affiliates. Put marketing restrictions into your affiliate agreement and notify partners immediately. It's your program--you set the rules. Or, if you prefer, you can let your affiliates run the majority of your internet marketing.