Great topic, I've also given this a lot of thought, especially after "incidents". Some thoughts:
Water containers need to be sturdy, so as to withstand transportation and be easily refillable and easily carried (like bladder systems). I would also include a small water filtration system or at the very least, iodine/purification tablets. Finding water isn't always so hard, finding *potable* water can be.
A handful of brightly-colored flash cards, a pencil, and a handfull of zip-lock backs can be an effective way of leaving messages.
All of my kits have a small fishing setup (line, handful of small hooks, couple of press-on sinkers). You don't need to catch large fish to get protein, and smaller fish are more abundant/easier to catch. You can live a long time on fish and water.
High protein, high calorie bars. Make sure you choose those with a long shelf life. Taste is not that important, but the packaging should be the sturdier kind, like PowerBars.
Extra socks, extra t-shirt. If you're stuck a long way from civilization and have to hump it back, you can wear the same clothes for a long time, but you want to make sure you wear dry socks, especially in humid places (may not apply to those of you in AZ, but back in South America, this was a mantra). Make sure the extra t-shirt is a bright, obnoxious color, or at least white, they're much easier to spot from the air than earth tones.
polished metal mirror, old school compass, maps, firestarter, Leatherman, kukri-style knife (can be used for chopping, and even for defense), soft broad-brimmed hat, sunblock, sunglasses, flashlight, plastic bottle of insect repellent, a handful of nylon zip ties, couple of survival blankets, rain/wind shell, cord, comprehensive first aid kit (fortified with vitamin supplements and adapted to the type of terrain/climate you frequent), one change of clothes (again, suited to your terrain/climate), 2-3 road flares, everything packed individually in the freezer-type ziplock bags inside a strong, comfortable backpack, preferably a brightly colored one.
One of the most important things about a survival kit is getting into a strict schedule of supervision/revision. Make sure you check it at regular intervals to make sure everything is up-to-date and usable. Label the survival kit as such and make sure that EVERY passenger knows its location. In a bad accident, you may not be the one able to reach it. Find a good location for it within your gear and ALWAYS keep it in the same location, you may have to reach for it in the dark, in a matter of seconds, under duress.
Sorry it wasn't more organized, I just wrote down thoughts as they came to me.
Editor, South & Central America
'99 UZJ100 Toyota Land Cruiser