* Larger ads outperform smaller ads. Wider ads also outperform more narrow ads.
* The Large Rectangle (336×280) is the best performing ad unit.
* Web users are accustomed to seeing menus on the left side of the web page. Move your menu to the right and place your AdSense ads on the left.
* Use the Google AdSense Heat Map to determine where to place ads. The darker spots are areas where users spend more time looking. Place your ads in the darker areas
Google has also has published a list of the highest performing ad sizes:
* 336×280 large rectangle
* 300×250 inline rectangle
* 160×600 wide skyscraper
Adsenes Heat Maps