Ad Network Etiquette?

What are some of the standards in the ad network industry?
What is the average contract length to be expected?
Is it typical to break contracts? If so, what are the consequences to ending your relationship early?
If you follow through your entire contract, and then leave to try out other networks, is it ok to come back to your original network if they worked best? Or is that looked down on by ad networks?
I would like to go out and experiment with different ad networks, but I don’t want to burn my bridges. I also don’t know what to look for when finding good ad networks.

See original post by JABacchetta