You should check out Google's Doubleclick service: http://www.google.ca/doubleclick/publishers/small-business/

Instead of just serving Adsense ads, you can set up various kinds of bidding (or manually create ads to start with) falling back on Adsense when inventory 'runs out'.

Basically, it will end up being a manageable/overridable layer between your site and Adsense, which you currently use.