One approach I've seen, aside from zoning related laws, is to give ag land huge property tax breaks. Want to convert it to some other use? Expect a 90% increase in property taxes.

It's far from perfect since some large landowners run "hobby" farms to get the tax break while producing nothing of significance for the greater society. Other developers take the bet that they can get the land developed and sold before the biggest bills come due and it will be someone else's problem.