Where I have worked on this I have run into two problems. First is how to measure a steep slope. This sounds silly until you get into an on-the-ground situation. Issues are:
Slope is measured as the verticle rise over a horizontal distance. But what is the minimum horizontal distance? 100 feet? 50 feet? And, from where do you start measuring?
Also, there may well be soils in your location that are highly erodible even at lesser slopes.
The solution I like is to use the USDA Soil Survey Erodibility Index. For instance, control development over all soils with the Erodibility Index of IV-e and greater. This has a couple of advantages. First, it pretty much eliminates the "how to measure" problem. Second, it will include soils that are highly erodible on lesser slopes. Third, the soils are already mapped in the local Soil Survey. And finally, the soils are described in detail and can be verified in the field. An additional bonus will come when the National Resource Conservation Service (the old SCS) finishes remapping the soils via GIS as is going on in many areas.
To help address steep slopes, Bath Township, Summit County, Ohio has made conservation development permitted as a use of right while making standard, "cookie cutter" development a conditional use. Bath has a web site that contains its zoning resolution. Good luck.