tacodoc's list is a great beginning, i dont have much to add to the list, just a couple comments from personal experience and some advice.
i absolutely love my co2 tank...it's maybe 30 seconds per tire to reinflate a 35 from around 15 to around 30 pounds, with a fixed 150# regulator and standard fittings/chucks/valves. the trade is space and limited supply, but a 15# tank will last a long time and only has around a 6" square footprint in the bed. redundancy is always good--i will always bring the air tank, but i will be installing a dedicated compressor for the lockers and will install the lines so i can run lockers or inflation from either the tank or the compressor. i can see losing the compressor or running out of co2 in a bad place and i'd rather run certain and safe than limp home or bash the rig apart using momentum instead of technique.
make sure your hilift is WELL GREASED, and that you have a good, safe (for you and the truck) lift point. i discovered a few weekends back that i erred on the first count, but was saved by the second...because i hadnt been regularly inspecting/greasing/operating the hi-lift, the reverse lever disengaged but the pins wouldnt descend. fortunately i have enough room between the sliders and the doors, and the sch40 stays put on the hilift safely enough that i was able to climb back in the truck and roll off the hi-lift safely, without hitting sheetmetal, breaking the jack, or anything else. incidentally thats a field recovery trick sometimes, it's just rare when you can do it without trashing sheetmetal in the process, and it's much safer to lift the tires and put a traction aid underneath and then remove the jack rather than what i did. this little incident stemmed from me showing off for a girl and resulted in her exploding in giggles afterward. happens every time..."hold muh beer!" "hee hee hee, you moron!..." anyway...bring the hi-lift, and have a good safe plan for using it that youve tried first before using in the field, whether it's as a winch (i havent) or a lift/rolloff point (i had used it as a lift, this was the first rolloff).
always bring a few pairs of gloves, like three or four, and make sure at least one pair is heavily padded...if you need help, dont assume everyone has their own gloves, and whoever is messing with cables/straps/pushing/rock stacking will appreciate the padding (probably you). leather gloves are so cheap (like a buck a pair) i always keep a few pairs in the gear bag.
a pair of clear goggles with side protection are essential for trail recovery and repairs...like the gloves, bring a couple pairs for you and crew. ive gotten extremely lazy and extremely lucky only once, it only takes once but it shouldnt and if you dont get lucky youll always regret being lazy.
a folding mil surplus shovel (like 5 bux) is great for sand, snow, mud, dirt, and sawing roots. if you have the room, bring a fullsize shovel like the guys at expeditions west...they hang it off the bedside so it's easy to grab and doesnt take space in the bed. an axe and pry bar are good as well, i dont have an axe yet but always bring a 3' pry bar for moving rocks and wood.
bring sand ladders if you can fit them in the truck. at 5'x2' and 2" thick, the pair fit just within the bedsides and once strapped down, the spare tire sits against them strapped to the bed (vs sitting against the tailgate which makes it hard to open), and the co2 tank currently lashes to the ladders (this will change but works fine for now). make 'em easy to get to...youll thank yourself when you need 'em.
the hi-lift, pry bar and other stuff (like axe, shovel, whatever) sits safely between the ladders and the tailgate, and the gate opens without resistance. downside is this stuff is still loose, which isnt safe and i know it. all that stuff should be locked down with quick-fists or similar.
bring only the tools you know how to use and have spare parts to use with...ie dont bring a spare axle if you/crew dont know how to swap it or dont have the tools. if you break, limp back and get a flatbed. that being said, imho a very good thing to bring if youre way in the backcountry with a modified truck on rough trails is a spare upper & lower ball joint, spare limit strap, ball joint press, and way to support the truck. same thing goes for front driveshaft and halfshafts, know how to swap the halfshaft and remove the driveshaft so if you break a shaft or grenade the front diff you can limp back in 2wd low range with a buddy and a tow strap. some good insurance here is swapping to uniball uppers, getting a manual hub conversion with beefier halfshafts, a front locker, and learning how to drive to minimize breakage. again expeditions west has information on driving, look for a very british whitepaper on desert expeditions.
i can still fit a grill, camping gear and two mountain bikes, and at least four gas cans and probably 15 gallons of water in the bed alone with all this stuff. packing method is very important for the large & heavy stuff, particularly hi lift and ladders since you want to be able to reach them quickly and without having to unpack the entire bed, but all of it should be low and centered. some very simple tubework will let me put mountain bikes, gas cans, water and co2 tank in the front, lock the spare a couple inches off the bed in the rear and put the ladders underneath, and fit another pair of tanks to the side of the spare. also, with the spare in the bed, there's room under the bed for an auxilary tank, air, or water, or just some storage room to install a couple drop boxes through the bed, like for batteries or compressors, just have a sealed compartment with a deck plate on top of each, dropping down where the spare used to sit. now if i can just stay outta the damn hospital long enough to do all this!
whew...that was more than i thought. there's an old thread floating around with similar content, it might be in this forum back at the beginning (check your view forum preferences), i think the conclusion was bring a spare truck
. never go alone.
-sean