It will never happen but...what they ought to do is pass out one big trash bag marked "Dumont Dunes" to every vehicle as they come in. Then, when you check out...turn in a FULL bag and get a coupla bucks as a rebate on your fee. Sure, some folks would probably just use them for camp trash, but it might encourage a little more thought before littering, even with such a meager incentive. Now, what to do with the mountain of trash bags at the entrance?
A more practical idea...get permission to put up a dozen temporary signs (like those annoying little 'new home' signs that pop up all over Vegas every weekend). Once past the entrance, I didn't see anything about carrying out trash or littering. Awareness on the way in can lead to surprising results.