Here's a great article about ad placement on forums from Google. Even if you don't run a forum it makes good sense to look at things from a users point of view and anticipate how their eyes will move around your page. This makes sense for every action you want users to take on your site, not just ad clicking. Often, as business owners we are too involved in our own business to see it from a user's perspective. Google, of course, has a lot more research revenue than our small businesses, so I find it's usually a good idea to listen to them. Maybe it would help increase revenue on your site.