Blend them good and be tricky to place them on places you expect people to click more often (following the Terms of Service of course, because you don't want to risk your account for pennies). Also, Google or search here for "AdSense Optimization" and read the tons of useful information.
Blend it with the background, bring it into content area. Avoid typical ad places like header, left nav etc. Refer the heatmap for more info. But above all, you need traffic. Improving the site content and getting good traffic is the only way to make money. Optimization can help you only to a limit.