The When, Where and How Much
We found this article (http://www.mediapost.com/publications/article/154587/) when doing some investigating on frequency. With nearly 2 billion ad impressions analysed for the purposes of this research from Casale there were some obvious findings: above the fold ads were (7 times) more effective than below. But there were also some more in-depth findings such as above the fold and within the first or second screen of a user's browser window were far more effective.
Also, despite ad effectiveness dropping drastically after the first two screens, the study did find that repetition - frequency - does work. "Ads shown five times or more to a user were 12 to 14 times more effective than ads shown less than five times." One thing is clear - to be as effective as possible, you need to consider the when, where and how much.