If you're looking for a simple method then a few coats of grout should suffice. You can either paint it afterwards to give a better colour finish, or my preferred method of mixing paints into the grout for the top coat.
For a different finish you could check out the gorilla glue method i used for a couple of mine, this is a different approach to most, i believe tomcannon is using it on his latest build too. That method can be found by searching through threads started by me (don't worry, i've only started about five so it'll be easy enough to find the right one)
Another alternative for desert backgrounds is concrete. Haven't done it myself, but if you check out the zoo exhibit thread thats in progress in habitat at the mo you'll see it, and maybe get some inspiration too.