They pay you a percentage of the sales you generate for them. You don't have to spend time and energy creating your own product. No dealing with customer service, fulfilling orders, and the like. And some of them pay 50% commission.
Join only a few affiliate programs whose products or services match the audience you attract to your site and concentrate on providing useful, information-rich content that will generate traffic and sales.
The links to the affiliate programs should be in context of something that you have written, rather than making them appear as if the main purpose of the site is to get people to click on them. This means that when visitors come to your site, they should feel that the site is primarily focused on providing information on that topic, and that the affiliate links are just a small aspect of your site.
Don't push your visitors to the click to your merchant's site, make them want to click. And they'll click-through with pleasure, arriving at your merchant's site with an open-to-buy mind. All humans like doing business with a company who has been recommended to them by a expert whose opinion they have come to respect.
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Finding affiliate programs
Finding online merchants with affiliate programs is easy. Just visit one of the most popular affiliate program directories. This site lists hundreds of merchants who will pay you to sell their products. It has even organized them all for you into categories...