Texture is pretty straight forward as mentioned above.
For sand dunes - I will usually use extruded foam and cut sort of boomerang shapes out of it.
http://geography.howstuffworks.com/terms-and-associations/sand-dune2.htm
Once I have that rough shape done - I sort of mess around with arranging them on the board to create an interesting pattern and figure out any areas which need to be accommodated (pipelines, structures, ruins). I find it is easier to build those on the flat and cut openings in the dunes to accommodate them before I start to shape the dunes.
Anywho, once that is all sorted - I use my hot wire cutter with the taper set to give it a nice look (one side of the dune is normally fairly steep - the other side fairly shallow). Cut the tapers and then double check the arrangement and fitment.
Glue them in place, use a putty to blend the joints (Bondo is my choice for that - stronger than Spackle and good coverage for your dollar).
On desert boards - I usually use a 1/4" or 1/2" base of foam as well in order to carve things like dry river beds and oasis into it. Cheap fan fold insulation is normally enough for that.
After all of that is done - I triple check my structures. Discount stores are an excellent source of things to be lost to the sands of time. Pet stores and wedding cake pillars are handy too. I may need to cut things out a bit more to make them fit. Once I am happy - any permanent structures get glued in place and they are blended into the dunes and underlying geography as well.
Large structures I like to keep removable. It makes moving and storage a bit easier and you can switch things out to make them appropriate for different game systems (you don't want a pipeline running through an Egyptian themed fantasy table). To accomplish good fits on that, I will use something like Saran Wrap to wrap the large structure with - put it in place and blend up to it with the putties. Once it is dry - you can remove the structure, remove the Saran wrap and finish the structure separately. The cavity left behind forms a nice snug pocket for the structure when you put it back in place.
Anywho - now it is just a matter of finishing things with your textured paints and sand.