Ahhh... now we have hit the "Nightmare" of every diormaist. The only sure fire way to keep this problem at bay is to put your dioramas in a case or perhaps a curio shelt that is enclosed... but assuming that is not an option, i use a FEATHER..... not a feather duster, but a single feather. Once or twice a year, I take down each diorama, and use a low pressure air brush hose cleaning (and I mean low... like 7-10 psi.... then i use a feather to get into areas that are a little more stubborn. Tedious, but it works. I don't like using water, beccause it can interfere with some of the inks and other materials i may have used to finish off a diorama (ie, wall papers, paper signs, ink washes, etc.). 'Course the way to eliminate THAT problem is to coat your finished piece with a flat finish spray - but again, not always an option.
Keith