If your site is driven by WordPress, one plugin that has worked for me is AdRotate. I had some difficulty getting this to work due to a lack of documentation, but once I found the right tag to use for my ad, everything worked fine. Here's a clip of the code that worked:
AdRotate will permit you to cycle through a group of ads where you want people to see them. Remember: you know what's on your site, but a new visitor may land on a page and not know the answer they've been looking for is on a different page of your site. Cycling ads in the margin is a great way to boost sales.
Cheers
James