Google have added a heatmap to their Optimization Tips page to give publishers a few clues as to what positions on your site might be best for Adsense Ads (darker colors are best performing positions).
Nice how they designed this in ProBlogger Colours by the way
You’ll notice from the heat map that ads close to content obviously do the best with left hand side ads performing better than those on the right of the page. Google also give the following advice on this page:
‘Certain locations tend to be more successful than others. This “heat map” illustrates these ideal placements on a sample page layout. The colors fade from dark orange (strongest performance) to light yellow (weakest performance). All other things being equal, ad placements above the fold tend to perform better than those below the fold. Ads placed near rich content and navigational aids usually do well because users are focused on those areas of a page.
While this heat map is useful as a positioning guideline, we strongly recommend putting your users first when deciding on ad placement. Think about their behavior on different pages, and what will be most useful and visible to them. You’ll find that the most optimal ad position isn’t always what you expect on certain pages.
For example, on pages where users are typically focused on reading an article, ads placed directly below the end of the editorial content tend to perform very well. It’s almost as if users finish reading and ask themselves, “What can I do next?” Precisely targeted ads can answer that question for them.’
As I always advise those I do Adsense consulting with – its worth keeping in mind that every page is different. Whilst the above positions are a good general guide there is always an exception and its worth moving your ads around a little, changing design and size and tracking the results to see what works best for your blog.