Check out Smart Sports from the guys who did Smart Thumbs and Smart Tube
http://www.smart-scripts.com/?action=smartspots
For $50 it's a steal! I use the script over many, many domains. One license is ok to cover all your sites.
Basically you create an ad spot in the script, and place that code on your site. Then when you want to add/subtract or change banners you just do it in one place in the script, which aslo gives you the amount of views, cliks and so on. Very nicely done.