A lot of great info in here. It's almost impossible to be prepared for everything but taking a lot of these suggestions will go a long ways. I've had my share in the dunes and suffered a few days out there commiserating with others that were broken. One thing that almost always helps when you'll be out there a while is having food and drink on hand, even if you are too pissed to eat or drink it. I went for a "quick" ride early one morning expecting to be gone about an hour and didn't make it back to camp until about 1am... it wasn't pleasant. Now I don't leave camp without enough granola bars/jerky/etc to at least be comfortable for a night in the dunes.
Also, a cell phone charger - the reception is better than ever out there and between friends, this board, a few FB forums you can likely get to one of them and communicate to someone that you are broken down and what the situation is.... which means you need to know where you are at so get a map and learn the lay of the land... even something as vague as "a valley west of Brawley Slide" narrows down the search quite a bit... and if you do get or use a GPS be sure not to follow it directly... the dunes don't run in a straight line and following one will get ya hurt in a hurry - I carried one and I did exactly that on a parts run back to camp while the wife and kids waited with the broken buggy, too big of a hurry and almost didn't make it to camp!