SUMMARY: You can learn a lot from this Case Study, because let's face it, charities like Foster Parents Plan are consummate direct response marketers. They have to be. They are not allowed to lose money on a campaign.
Most fascinating fact -- they discovered that it's possible to have a "control" ad that keeps steadily, quietly succeeding for months on end, even as other campaigns' clicks flare up and die down around it.
We've known controls are possible for mail -- but for online banners? Here's more info, and plenty of samples: