The tricks and plan well in advance
Do take account of Lhasa's altitude, at 3660m / 12,000ft, and the potential for altitude sickness. Here are the tricks to avoid, or at least lessen, the effects of the high altitude.
People flying in without prior altitude acclimatization will likely suffer headaches and uncomfortableness for a couple of days, the choices are take to acclimatization drugs or take a more leisurely, adventurous journey in, eg the new classic train trip.
There are many options for getting to Lhasa however the major consideration should be acclimatization to altitude as Lhasa is at 3660m/12,000ft (not 3490m or variations on that). I have seen that the altitude there causes more initial problems than Leh, India, which is at 3480m / 11,400ft.
If you want to get to Lhasa as quickly as possible then the best plan will be to take Diamox, a drug that kick-starts the body's natural acclimatization process. We suggest a 125mg dose three times a day starting roughly one day prior to beginning your international flights. Since this drug does increase urine output that is why starting well before you fly is better. Take a look at our full health discussion that includes the side effects.
Respecting altitude is all about taking time to go higher, rather than the actual altitude, and this can be summarized as from ~3000m/10,000ft only ascending around 1000m/3300ft in sleeping altitude over a period of four days. Our itineraries are planned around sound acclimatization.
You gain useful acclimatization with a few nights at over 2000m/6500ft, although even Kathmandu's altitude of 1350m/4400ft seems to make a small bit of a difference, and you could theorize long flights where the cabin pressure is around 2200m/7500ft for much of the flight, might make a difference too.
There are a surprising number of flights to Lhasa (Wikipedia) and airline options from from all over China, so from Beijing, Shanghai, Xian, Chongqing, Chengdu, Kunming (only one a day) and a few more obscure places plus Kathmandu.
With the altitude of Kunming a couple of nights there works, however although Kunming has a busy domestic airport, there are fewer international flights that you might expect. Xian has the amazing terracotta warriors; Chengdu has pandas; Beijing offers the Temple of Heaven and more, if you have a day or two to add on...
Expedia and other internet services perhaps the best way to buy tickets as part of your international ticket. For less known airlines you can try booking direct but am guessing your chances of success are middling, or our Tibet agent can also book for you, with plenty of advance notice.
Taking a train across China is a great way to get a quick taste of China, and with a stopover in Xining 2200m/7200ft, the best way to acclimatize too. There are direct trains to Lhasa however we recommend the Xining stopover. This also conveniently broadens your international flight and travel options and allows extra recovery time. You can book tickets yourself or our agent can assist for a small service charge; note train tickets are best booked two months in advance and do actually fill, sometimes a month or more ahead of time.
First begin in a convenient place to fly to: Beijing, Xian or Chengdu and then take a train (or domestic flight) to Xining. There are plenty of train services so you can usually arrange to fly in and go directly to the train station and hop on a sleeper train, rather than needing to stay overnight anywhere. Slick.
Then spend two nights in Xining and then take one of several trains to Lhasa. It is possible to spend just one night in Xining however then definitely take Diamox (the acclimatization drug) while in Xining and en route to Lhasa.
We have more info on Xining we will send you and we arrange a hotel and program there for you, including a zoo with snow leopards.
The best class is first class or soft sleeper, with 4 berths to a cabin, and the cabin has a door that can close however there is normally only one carriage per train of this class, and tickets book particularly quickly, or are entirely unavailable, booked for government officials. The next tier is 2nd class, what used to be called hard sleeper, and is no longer hard and is certainly comfortable enough. There are six bunks per cabin with luggage going under the lower bunks and there is a luggage area too, although you still want to travel with tidy luggage that you can carry yourself, and that isn't too, too big. A large/XL duffel with shoulder straps should work well enough.
Many Tibet treks and expeditions start in Kathmandu for the convenience getting a group visa plus the Tibet permits there, getting last minute gear, and as many Tibet travel agencies didn't write English, bookings were made through Nepali travel agencies. However it is not the only option and plenty of standard outdoor gear is available in Lhasa (as you would expect as most of it is made in China). Although you are able to buy Lhasa-Kathmandu flights online, for reliability it is slightly better to buy them through our Nepal agent as sometimes internet ticket holders are mysteriously bumped off.
The standard and only route used to be driving three hours to Kodari on the Nepal side and crossing the border to Chinese Zhangmu ~2200m, staying the night there, then driving only a couple of hours to Nyalam at 3750m and spend two nights there for acclimatization. The next stage crosses a 5000m pass and on the other side, on the Tibetan Plateau proper, there's a choice of destinations, although two nights at dusty Tingri at 4350m was standard for Everest and Cho Oyu expeditions, and for overlanders visiting Everest Base camp. This itinerary of five nights between Kathmandu and Everest Base Camp worked well.
Now, after the 2015 earthquakes, the Chinese authorities have partially closed Zhangmu, due to the outburst threat posed by a large glacial lake, that if it breaks its terminal moraine, would destroy the gorge road and much of Zhangmu too. So there is a new border crossing area near the Langtang trekking area, through the Nepal district of Rasuwa and in to the Tibetan-Chinese area of Gyirong (Kyirong). The road and facilities on the Chinese side were finished in 2011 however, as you can probably guess, Nepal was late to the party and never finished its side of the bargain. The earthquake damage was severe on both sides of the border however now (Sept 2015) vehicles can cross and this route will open to tourists in 2016. The issue is, as always, acclimatization, and this crossing will require a few days at intermediate altitudes before heading to central Tibet.
At the end of the trip you could take a train back to where your international flight starts from however flying makes more sense now that you are not allowing for acclimatization. Do buy the ticket well in advance. You can book this (usually separately from your main tickets) or our agent can. You must take a train, flight or package tour out of Lhasa (or our Yuzhu Peak trip?!). You cannot travel on public bus.
Do feel free to discuss travel options with us too.
Be aware China still has a rule that foreign tourists can only stay at tourist-class hotels; only some hotels are licensed to accept foreign guests. They are strict about this unless perhaps you are from Singapore/Hong Kong.
In tourist centres some people speak English however not train staff, although other passengers can often assist. Traveling out of main centres can be hard work. Also understand in China your passport number is your ID, and you are a number, more than anything else, your passport number must be correct on all documentation. If for some reason you change your passport, bring your old one, or at minimum, a copy of it.