It might be tempting to hide your ads by making them the same color as your background or camouflaging them on areas with busy background images. You do still get paid for pageviews, so invisible ads would still pay, right? Don't even try it. This violates Google's Terms of Service, and it's easy to get caught.
Don't stuff your ads far below the rest of the content, either. Clicks pay better than pageviews, so it's to your advantage to have your ads prominent. Try to make the ads look like they belong on your page.