Thats right,
CrazyEgg lets you find the right ad placements.
How? Sign up for a free account and submit one of your websites to the heat map. They will track your visitors cursor and where they click.
At some places they may hold the cursor a lot of times, and that area will heat up.
If you analyse the outcome of crazyegg good. you know where your best ad placements are and probably can earn more !!
Just try it out.