It is no more a hard thing to travel to Tibet for foreign tourists. You can take flight or train from many cities in China such as Beijing, Chengdu, Xi’an, Shanghai, Guangzhou, etc. For the moment, the most convenient places shall be Beijing and Chengdu. Xi’an is also a good choice but due to the limited flight (usually one flight only and not daily) and limited tickets of pass-by trains, it is still not easy to make connection there. You can also take a direct international flight from Kathmandu to Lhasa but which is not suggested except you have a tour there. According to a memo between the Chinese and Nepal governments, foreign tourists must obtain their China Visa in Kathmandu if they would like to enter from there, no matter if they get the visa already in their home country or not. This usually requires three days at least.
No matter which way you take, a Tibet entry permit is necessary for issuing the flight/train tickets.
This permit is released by the Tibet Tourism Bureau to officially licensed travel companies only. You shall provide a copy of your passport and visa with your Occupation info for the permit application. It usually takes five working days but we shall always try to apply for it as early as possible to secure a smooth trip since more and more travellers coming to Tibet making the queuing much longer than before. The cost is around 7USD charged by the government department. If you will travel outside Lhasa and some remote areas in Tibet, some other permits will be applied as well including the Alien’s Travel permit, military permit, etc.