First things first, the positioning of adverts on your site can make a huge difference to your revenue potential, the heat map on googles FAQ here is a very good guide as to where best to place your ads for maximum clicks. Be careful not to put the ads ina location where they may discorage teh person from staying on, or revisiting your site.

An banner on top, an skyscraper on the right, and a large rectangle below seems to be a popular option, however there are many variations, and it is upto you to find the one that works best for your site. Below you can see where the best ad locations are. This is based on Googles Adsense statistics so is fairly accurate, rememebr though, it depends on how the ads look in your site as to how effictive they are.