So, all the conventional wisdom I've read says that the king of ad placement is the 728x90 ad above the fold near the content. After a surprising and frustrating amount of work to make that happen on a WordPress theme, I see that the two ads on the right sidebar [300x250, placed just below header and another further down] outperform the leaderboard by a magnitude of four. Any ideas why that would be? What am I missing?
My 300x600 ad on the right out performs my 728x90 ad on the top as well. I would imagine because it's closer to the content and when a visitor completes the article, the ad is right there, ready for the clicking.
I always found that my 300x350 or 300x250 sidebar spots always did much better that any 728x90 or 468x60 placements ever did.
Leaderboards never performed well for me either. However ads within the page where the main content was almost always outperformed any sidebar ads.
Ad size and placement depends on many factors and usually different for different type of website. You should try it your self by adding code in different position, and finally choose which work better. For your experiment I think this will help you.
You simply have to test before you take a decision. If you change it over and over you will not see where the best position is.