Agree, with [MENTION=638610]JPDM[/MENTION] that the belief that the train to Lhasa helps you with acclimatization, is not correct.

If one has the time, probably the easiest and best way to access Tibet and properly acclimatize, is to fly to Kunming, Yunnan (1900 m) and spend a day or so. Then work up with a few days in the Lijiang/Tiger Leaping Gorge area (2400 m), then go by road to Shangri-La (Zhongdian, 3200 m) for a couple of days, then fly nonstop from Shangri-La to Lhasa and one should pretty much hit the ground running. Yunnan has plenty of sightseeing to do including these locations.

Alternatively, going from Chengdu or Lanzhou or Xi'an to the Xiahe area (Gansu uplands) and acclimatizing in that area for 2-3 days then flying nonstop from Gannan airport to Lhasa, would also work pretty well though the nonstop flights aren't daily.

But of course, most visitors are in a rush.