The Internet has been around long enough now for some online marketing principles to emerge as proven success drivers.
Most people buy on their third to fifth contact.
Which means you have to get their email address the first time they visit your site, and periodically send them some useful free information to get them to return to your site several times. You have to give them a valid reason for them to give you their email address - no one gives their email address without something worth it for them in exchange. Obviously, providing good and relevant free information is very important here, you just can't email pure advertising to people. Many sites that do not do any other form of advertising report affiliates as an even larger contributor of their total sales. Having an affiliate program gives you the ability to convince other related sites to link to your site.