WATER! BRING LOTS!
Miscellaneous: when making the trip to xigaze (usually from lhasa), you'll be on the road for several hours. the places along the way where you can stop will be few and far between so make sure you bring an adequate supply of bottled water (adequate depends on how quickly you get thirsty).
this area of tibet is very dry (you'll even see sand dunes along the way) so better to have bottled water to drink as your 4x4 negotiates the seemingly endless hiway.
bring snacks to munch on as well.
WATCH W/ ALTIMETER (it's all about elevation)
Miscellaneous: bring a watch with an altimeter (i had a suunto xlander).
if you're going to tibet, you'll be encountering elevations you're likely never to encounter again - personal altitude records is what i mean. might as well record it, right?
also, better to be aware of the elevations you reach so that you can better "hike high, sleep low" - the general rule in traveling at high altitude so as to avoid acute mountain sickness (AMS). being aware is key if you're going to stay healthy up here.