"Cotton kills" is one of those rules of thumb that will usually keep you alive, but doesn't tell the whole story. Cotton kills when you are wet and cold. It completely loses any insulation value and as water or sweat evaporates it carries away your body heat. Between about 30 and 55 degrees Fahrenheit its enough to turn cool weather into fatal weather. In hot climates, its perfect. Long, loose sleeved cotton shirts and pants will keep the sun off of your skin and slow the evaporation of sweat. This will keep you from burning (although you can still burn through clothing) and keep you cooler while also keeping you from dehydrating as quickly. People will think you're crazy for wearing long sleeves and trousers in the desert sun, but you'll actually be much more comfortable. The US Army's DCUs and BDUs operate on this principle. You loosen the cuffs and wear thinner fabric in the summer versus the winter, but you still wear long sleeves. Cotton or a 50/50 cotton/polyester blend will work fine. For pants, BDU pants in solid grey or tan/khaki will probably fit the bill for you. I can't think of a specific shirt off the top of my head though.

When it cools off at night, just add some light layers. A thin layer of silk or polypro long underwear shirt will warm you significantly when its not too windy, but they'll take up almost no room. If you need more carry a wool or acrylic sweater or a light fleece jacket. Silk, wool and synthetic fleece, acrylic or polypro will all keep a reasonable amount of insulation value even when wet.

For rain any good, unlined rain suit will be perfect. Something you can put on quickly while still wearing your boots.

Hope this helps some.