I tried this on one of my sites, and it is has been working very well, so I thought I'd share it.
On my most trafficked pages, I added an affiliate link. I made the font size rather large, and the text rather colorful. I placed the link in a prominent place within the body of the text.
There are no other links on the pages, other than the site's internal navigation on the left hand side. The affiliate link just jumps out at the user.
I used my own text for the link, wording it so that it sounds like something the user just has to have (but it is truthful about the product).
Essentially, the user only has three choices: go to another page on the site (where they will likely encounter the affiliate link again), click the affiliate link, or leave the site completely. With the affiliate link being the most prominent, and the most enticing, I'm getting a huge conversion rate.
The point is, if you want something to convert (whether it is an ad, a form to fill out, an affiliate link or whatever), make it prominent, make it enticing, and make it be the one best choice on the page.