Go with a brown basecoat, or black spray base then a painted-on brown base. Then use a slightly lighter mix of brown fan/dry-brushed on heavily. Then again, slightly lighter.
Last, a fast drybrush of white/bleached bone.
So, base dark brown (scorched or bestial).
Thick fan/dry-brush of lighter mix of brown.
Again, even lighter over previous.
Drybrush bleached bone mixed with hint of white.
Maybe even do a couple corners with a slight dab of white drybrush alone.
I'd keep the sand to around the base of whatever it is, whether rock formations or structures. Use PVC Glue, or Elmer's-eqsue watered down a tad and placed where you want the sand, sparingly. Shimmy it in the sand til how much you want sticks.
I used all of this on my own desert terrain. It came out amazing. Just make sure to use your imagination of what it SHOULD look like and modify the mixes and colors from there.
Oh, and to make little dunes against structures, just either keep glueing over the previous sand and re-adding, or glue a small piece of plastic or card there at the right angle and add sand to that.