Basically, an easement is an agreement between a property owner and a representative of a historical preservation society. (In fact, a couple of the state programs you'll run across fit into this category.) In exchange for your promise to maintain the property's historic character, you can receive fabulous benefits in the form of reduced income tax, estate tax or property tax.
Remember that an easement can be forever. Once signed, sealed and delivered, it's yours, and it's filed as a deed with the title of your property, passing down to any future owners. This may be beneficial -- what potential buyer doesn't like reduced fees and, maybe, the thought of living in a verified historic building? But it might also scare off future buyers who had their hearts set on converting your Victorian into a Gregorian (or worse!).
Preservation societies funding easements exist at the city, county and state levels, and they can be found easily through a quick Internet search.