Tibet - Health
Keep in mind that you are traveling to and in a high elevation. Don't overestimate your body's capacity to adjust; plan enough time at the beginning to get used to the difference. Especially those traveling by plane may find the thin air problematic. But even buses hold their treacheries, for the passes on the drive to Lhasa reach elevations of over 5000 meters. Consequences of the height difference can include headache, nausea, and vertigo. The best preventative is simply to be physically fit.
Dogs in Tibet
In addition to the usual vaccinations, consider one for rabies. Tibetan dogs (except perhaps in downtown Lhasa) are constant companions. They are used to people and spend much of their time lying in the sun, but they are really everywhere, so that one has to be careful not to step on a dog when walking around a corner.
Sunburn is a problem that's often underestimated, especially in spring and autumn. Definitely wear sunscreen and clothes that cover the entire body to protect from the heavy solar radiation. A hat is de rigeur in Tibet.