Meant to post this awhile ago -comes from experience working in the alkalai flats for weeks at a time, not just going to the event. These are some things I'd regard as absolutely crucial that I haven't seen on other folks' lists, especially as the kind of collapse that would justify all these preparations is most likely to be accompanied by some serious desertification.

A good, tough, widebrimed hat.

Extra pairs of sunglasses.

Steel-toed boots.

Socks - lots and lots of socks. very important. Other clothes you can wear over and over again with not much worse consequences that hte smell, but you do NOT want to be going around in used socks day after day6, especially if you've got the above mentioned boots.

SPF 50 sunscreen, and lip balm. No chapstick or other petroleum based lip balm - that stuff actually dries your lips out and you'll keep needing to apply it. Something like Burt's Beeswax.

A water bottle or canteen (plus a few extras). You'll need to carry water around at all times, assuming you can't trust water you haven't purified yourself.