If you run a web site of reasonable popularity, you probably get invitations to sign up for ad networks from time to time. They want you to try them out, and will sometimes talk a great talk about how well they will do.
I always tell them “put your money where your mouth is — guarantee at least some basic minimum during the trial.”
Most of them shut up when I ask for that, indicating they don’t really believe their own message. I get enough that I wrote a page outlining what I want, and why I want it — and why everybody should want it.
If you have a web site with ads, and definitely if you have an ad network, consider reading what I want before I’ll try your ad network.