cotton, cotton, cotton! linen is also well adjusted to that heat the desert provides. light colored, lightweight skin covering clothes are the norm. they do get storms once in a while, so an umbrella couldn't hurt.
Toiletries and Medical Supplies: Water, SUNSCREEN, 'sports drink', WATER, ALOE VERA, water, aspirin.......
Miscellaneous: If you must park outside on a hot day: crack your windows to let your car 'breathe'. Use a sunshade in the front dash (and put one in the back window if you have an extra!). A white towel over the steering wheel could be helpful, as is a cloth or something to protect your hand when trying to open the car door on a really hot day (carry a cold drink to the car as extra insurance).