It is possible to visit Lhasa on 3-7 day tours from Kathmandu, Nepal, but there have been reports of tours that do not allow enough time for visitors to adjust to the dramatic altitude change resulting in some travelers sufferring altitude sickness being left off along the way (without any refund, of course). You can choose from the options fly-in and fly-out, drive-in and fly-out, etc. Fly-in fly-out comes at a small extra cost and offers most confort and safety.
Chinese Standard Time (Beijing) is used in Tibet, which is 8 hours ahead (+) of GMT and 2 hours 15 minutes ahead of Nepal. However, it is not uncommon for Western climbing groups to keep on Nepali time since this better coincides with the expected times of sunrise and sunset.
Non-Chinese nationals are required to obtain a special permit to visit Tibet. Individual permits are hard to obtain, group permits (at least 5 person) are easier. Travel agents from Kathmandu are very good in obtaining one and also a Chinese visa for you in very short time (one day or two) when you book a trip.
The Lhasa Gonggar Airport (贡嘎机场) (IATA: LXA) is about 100 km from Lhasa. It takes 2 hours to the center of Lhasa. There are flights from Beijing, Chengdu, Chongqing, Guangzhou, Hong Kong, Kunming, Qamdo, Shanghai, Xi'an, Xining, and Zhongdian (Shangri-La),
International flights are available to Kathmandu, Nepal.
Buses run from Golmud in neighbouring Qinghai province, but are almost as pricey as the flight from Chengdu due to the permit issue.
The Qinghai-Tibet (Qingzang) railway connects Lhasa and Golmud, with services continuing onto Xining, Beijing, Chengdu, Shanghai, Guangzhou and Chongqing.
Total Trains Running To/From Lhasa
T27/8 to/from Beijing (West) - Lhasa - takes about 48 hours
T22/23/24/21 to/from Chengdu - Lhasa - takes about 48 hours
T222/223/224/221 to/from Chongqing - Lhasa - takes about 49 hours
T164/5 T166/3 to/from Shanghai - Lhasa - takes about 51 hours
T262/4 to/from Guangzhou - Lhasa - takes about 60 hours
K917/K918 to/from Lanzhou - Lhasa - takes about 28 hours
N917/N918 to/from Xining - Lhasa - takes about 24 hours
It is very difficult to get a train ticket during January, February (Chinese New Year) and July and August (summer holidays). Rail travellers may want to wait several months until more trains to Lhasa are ready.
Also you will probably get ripped off on arrival at Lhasa station by the taxi drivers who will not use their meter (starting rate of Y5 and then 1.8km after the initial distance covered in the Y5). The normal rate should be Y40 but sometimes they want Y100.