The number one thing you will want is probably a straw hat with large brim. Must be straw or mesh to breathe. You may be able to find packable ones. Really, it will keep you cooler by keeping the sun off your head.
People who work in the sun also sometimes put small towels or light cloths under their hats that covers the back of their necks. I don't care for the look of this, myself, but what the heck. I think this keeps you noticably cooler if you keep the cloth wet. Evaporative cooling. Otherwise it just keeps you from getting a "red neck". You may be able to buy a cloth hat with a flap that comes down over the back of your neck.
As poster above said, light colors. No polyester. In the heat polyester is like wearing a plastic bag. 100% cotton only (or silk will breathe too, but you don't want silk).
Consider somewhat gauzy or flowing clothes. Not tight, not heavy. Think of the traditional white robes in the desert. Under hot, hot sun the white, very loose clothes keep the sun off you while keeping the air circulating. In fact, many of the groundskeepers in this area (90-95 degrees in the summer) wear long sleeves all summer. I assume because it keeps the sun off their skin.