A detailed Google Webmasters Help thread has discussion around how Google perceives affiliates and how they need to go above and beyond to warrant ranking well in the Google search results.
Googler, JohnMu, answers one affiliate, as to why he is not ranking as high as he would have liked:
I believe one of the issues that you might want to work on is to make sure that your site provides unique and compelling content to the web. It's important for us that the site not only contains "rewritten" descriptions and articles, but that it actually provides something more than just "different words" for the same content. In other words, given the choice between your site and the company providing the products, what could your site offer to make Google want to show your site instead?
John then links this webmaster over to a Google document on Affiliate programs for webmasters. The document explains how you can make your site stand out from the other affiliates:
- Affiliate program content should form only a small part of the content of your site.
- When selecting an affiliate program, choose a product category appropriate for your intended audience. The more targeted the affiliate program is to your site's content, the more value it will add and the more likely you will be to rank better in Google's search results and make money from the program. For example, a well-maintained site about hiking in the Alps could consider an affiliate partnership with a supplier who sells hiking books rather than office supplies.
- Use your website to build community among your users. This will help build a loyal readership, and can also create a source of information on the subject you are writing about. For example, discussion forums, user reviews, and blogs all offer unique content and provide value to users.
- Keep your content updated and relevant. Fresh, on-topic information increases the likelihood that your content will be crawled by Googlebot and clicked on by users.
Does this mean you must do this to rank your affiliate site well? Not necessarily but for the long term, it might be best to follow these strategies.
Forum discussion at Google Webmasters Help.