You Really Do Need It All
by D_Shannon
Backpack with water/dust-proof cover.Fanny/day pack with room for water bottles.
We packed for everything from sunstroke to snowstorms, and we needed it (went in September): fleece jackets, shorts, tank tops, hiking boots, sandals, pants, plastic ponchos....and a khrama (a cotton Cambodian scarf) to wind about your head - keeps the sun off, the sand out, and the wind from ringing in your ears.
Between the three of us, we had enough medicine to cure diarrhea, constipation, gas, motion sickness, PMS, headaches, backaches, knee aches, lactose intolerance, dehydration, and the common cold.
No stores in the desert. Bring more than enough film and extra batteries. Consider keeping your camera in a ziplock to keep it sand-free.
You need a sleeping bag, even if your tour company claims to be providing one - if it's cold, you'll appreciate having two.
Duct tape, hand mirrors, spices,...