There will probably be a lot of different answers to this question....but here goes mine.
Sand and kitty litter.....that's right......the stuff cats poop in.......make sure it's not the used variety though

buy a small bag of it just for this purpose...or if you already have some for your cat....use a small amount out of the bag.....mix the two together......Sand + Kitty Litter....now pick your base.....a piece of wood let's say....I would start with a base coat of Celluclay mixed with water and a lot of white glue....spread it over your base....then when you're happy with the shape and size.....sprikle the sand/kitty litter mixture over this fairly heavily and gently pat it down to emmbed some of it into the Celluclay....if you have any vehicles in your scene...now is the time to push them into the ground work to make the indents for them to sit in later.....then remove them....now set this aside for a day or 2 to dry.....and blow off or dump off the excess....if there are bare spots you can make a mixture of white glue and water about 50/50 and put it on those spots and sprinkle more of the sand mix onto it...let it dry and so on and so forth.
Go into the Gallery and under Dioramas look for Folgore's folder.....he has done the nicest desert diorama I've seen in years......study how he made the ground look....you can't go wrong when looking at a master's work.