No matter which way you take, a Tibet entry permit is necessary for issuing the flight/train tickets.
This permit is issued 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 quequing much longer than before. The cost is around 7USD charged by the government department. If you will travel outside Lhasa and some remote areas, some other permits will be required as well including the Alien's Travel permit, military permit, etc.
About the Author
Sherry Jia, experienced Tibet travel advisor from www.toursoftibet.com, a local travel agency based in Lhasa with an office in Chengdu to handle all the on-line inquiries regarding a tour to Tibet and China. There are day-to-day itineraries of some popular tours in Tibet including that to Mt. Everest Base Camp (http://www.toursoftibet.com/docc/TP.html), Mt. Kailash and some trekking tours as well.