As Kitchen Designer rightly points out, another interesting technique that leads to higher click-thru rates is the way you format the text of the Adsense unit.
You have control of the following variables:
Try setting the background and border to the same color as your site background. Next, change the title to the same color as the links on your site. In terms of click-thru, it is best if you use one consistent link color on your pages for both the Adsense and your navigation links.
Finally, make the text and URL color the same color as the text on your site. This technique helps you blend the Adsense creative into your site, effectively blurring the lines between advertising and content in the mind of the visitor.
Combined with appropriate Adsense creative positioning, the text variables are a powerful way to influence click-thru percentages.