I had adsense banners on my proxy for 2 weeks and never got one single click. The proxy was centered in the middle with one banner above the logo and another banner under the browse button. I decided I would try to change the look and put the adsense banners in places people said work a lot better. I made it so the banners would go directly above the URL bar and right above the browse button and got 5 clicks for $4.50 the day after it was changed. I never thought it would make a difference but im happy I tried.
I had a similar experiment with one of my sites.(site directory) I was placing an google ad(with 3 links) just above the directory links. After 1 month i placed the google ad 3 rows lower. The CTR just doubled. I am changing the ad placement every month and seems to get results for me.