In Ann Arbor, the City and Township have a special millage that all the residents pay, and the funds are used to buy development rights for farmland on the outskirts. I believe the City has a department that evaluates and ranks all the potential pieces of land to determine which ones get purchased. Of course, the landowner has to be willing to participate, too. It seems to work, but its rather controversial. Its detractors say that all it does is push the sprawl out another ten miles, which seems like a valid point to me. But this seems a lot more workable then drawing a line on a map and saying "no development on the other side of this line". You gotta give them credit for trying something, though! Sorry, I'm getting a little off topic.