I used Brak's tutorial to get the detailed areas (the cracks and corners and such), then I used dust from my back porch and hairspray for the final details. Since I live in the desert, its the real deal.
Funny thing though, when some guys from our garrison and Art from TDH went to the Imperial Dunes (Imperial County California) for a photo shoot, the wind created a sand-blasting effect and actually cleaned our armor back to white. My boots were the dirtiest part of my suit and now they are almost back to clean TK status.
It kind of makes you think that the "actual" sandtroopers would have been pretty clean too (except for their filthy brownish underarmor)
Steve