When a search engine crawls (reads) your webpage it puts a lot of weight on the text and text links. Text plays a major role when a search engine decides if it wants to index your pages (include them on search results) and what search terms or keywords your site will appear under. 

Text links are typically top performing ads for publishers, but there are a couple of steps you can take to maximize their effectiveness.

First, place your text links in relevant content that includes your keywords. Emphasizing keywords throughout your page will help to insure that the search engines index pages containing your affiliate ads.

Second, avoid using phrases like "click here" that don't have any meaning to the user. Use one or multiple keywords as the link itself. For example, if you want your page to appear when someone searches for "blue widgets", having text links on that page that contain "blue widgets" will greatly increases your chances of appearing high on the results.

Also include product names, descriptions and other keywords close to the link. Not only will this help with search engine optimization, visitors will be more likely to click your links.