I spend lotsa time in the deserts around SoCal. The pictures and info Plaza provided are a good source. The desert is a mix of no-vegetation, medium/average disperesed vegetaion, or rich deposits of desert vegetation found in particular sweet spots. Try more vegetation on the higher altitudes and on the mountains for your terrain is my simple suggestions. Not really accurate though if ya want to get into it deeper, but probabley the best you can do in AH for representing mountains and hills that are one-half sand-blasted by the wind and the other-half is more fertile and protected from those strong winds.
The desert terrain in AH looks better in flatter and low-lyeing areas (on lower/rolling hills). The winds in the desert flats (open flat areas) are very ferocious, resulting in a lack of vegetaion. And water, even though the dessert lacks it, is still what dictates how well vegetation grows in certain areas (especialy areas that the wind doesn't dry-out to bone almost imediatley when it kicks back up after a brief shower/storm). Next to the coast, either wind and currents help create a "rim of life" that's thick with vegetation and wildlife that's supported by coastal moisture and fog that rolls in to deposit some moisture, or the dessert litteraly is steam-rolled into the ocean due to winds blowing sand, dry heat and any potential coastal rain/mosture away from land and into the ocean.
Perhaps some satellite photos of some dessert regions would be of some great assistance here.
IE1: Here where I live it's been very metropolinized, but still from satelite imagery you can clearly depict the fertile regions between the coast and the large mountain chain that runs down along the coast inland. Just inland further of these mountains the desert begins, you can pick out more fertile desert in the regions closer to the mountains, be it runoff through streams or simply shelter from the winds.
IE2: Saudia Arabia - The SW coast is fertile, fed by moisture blown in from the ocean and Africa. As those currents and winds work their way North East though, over some mountains/hills and twords the NE coastline, they get dryer and dryer, and there is no real fertile area along the NE side (compared to the SW coastal region) as the desert sands and dry air are blown out to sea. Note how to the north the water-rich rivers that run through Iraq create a fertile barrier/strand against the dessert, before it continues rolling with the winds into Iran.
