Less is more.
Blend the ads into the site, beginner temptation is to make the ads stand out but studies have shown these gets less clicks, blend them.
Putting a graphic directly above of below and ad can increase click through rates - I think this may be against Google's T&C's now though.
Look at some studies on how people view websites - people usually start at top-left and scan right and then look down the left hand side. So top-left and top middle is best place, bottom-right the worst.
Suck it and see. Put some ads on and see how they perform for a few weeks. Then move them around and compare the results in another few weeks. If I remember rightly the Adsense site has some features to help with these split-tests.