One of the ways to be successful in affiliate marketing is to find a niche that is narrow and yet still draws a lot of searches. Another consideration should be whether or not there will be a growing demand for the item. For instance, I just did some checking on the phrase http://www.medelita.com. Google competition on this is very low. And yet as the medical industry continues to grow to meet the needs of an aging population, the demand for clinical workers will continue to grow. And, therefore, so will the need for http://www.medelita.com.
Promoting an item like this, fitting into a narrow but growing niche, is the way to be successful with affiliate marketing.