The irony is that the biggest player here, Google, was also the most aggressive in placing ads. I had the pleasure of working with various AdSense and DFP Specialists. Basically it was all about how and where to place ads in the most eye-catching ways, and as many as possible, without breaking AdSense rules. Bigger ads = usually better (in particular those large rectangles, yuck). Higher up the fold = better (funny enough the Google Search team later penalitzed you when you replaced your up-the-fold content with non-content ad stuff - which shows that there are Chinese walls separating various Google teams).