If you use Google AdSense as a source of income on your website, you probably want to know where to place the AdSense code on pages to get the best results. This is a topic often discussed AdSense publishers and there were some interesting results after numerous tests. First, note that you can display only two blocks of ad code on your Web page. For best results, use both, both AdSense for search, every page. The AdSense for search box can be inserted anywhere, but apparently it works better at the top of the page, just below your header, left or right. There are many debates about the AdSense code. Some argue that the format Skyscraper placed on the left side works best, while others maintain that it is rather on the side. Make sure you insert this code or the right side or left side - but not both. Remember that the number of blocks available is limited. The AdSense code must also appear in the section of the page content. Many swear by the position above the page content. Others have found they get better results by putting the ad code in the middle of the page content. Whether you put it above or in the middle, use the rectangular format that fits best to your page design. As you can see, there have been many attempts to achieve, but everyone gets different results. You'll have to determine all of this yourself by making your own tests. Put the code on the left side for a week, observe the results, then move it right. Post an ad block above the content, then observe the results if placed in the middle. Most people agree that the ad code at the bottom of the section of content does not work very well. In addition, watch your page and look where your eyes arise. Ask others to watch and learn the path of their gaze. Where ever there are the looks, this is where it will place the blocks of AdSense ads!
i used to run google adsense a while ago on my blog, and for me placing a 336x280 above the fold, just bellow my article titles used to work best... another thing that i've noticed is that having a darker background helps, as it makes your visitors concentrate on your content. for link colors i was always blending my adsense ads links with my site link colors... and also my sidebar articles were somehow looking like google adsense ads... but that doesn't have to work for everyone sometimes making your ads stand out from the rest of your website can generate a higher CTR...
I always follow this simple rule: the best location for adsense is where I'd like to have my own ads if the site wasn't mine.