Well, I know you said that you realise that it is by banners and Google ads but I guess I will reinforce that.
I have been using Google ads for sometime now and it has been great.
I let users place the banner where they want to on their page, freedom helps.
If you have a dedicated server, I think your best bet would be to get mod_layout which lets you modify automatic headers and footers.
I just make sure I enforce that people are placing banners on their pages but I tend to be very lenient.
Good luck!