There are main cities to enter Tibet, such as Beijing, Chengdu, Xi'an, Chongqing, Shanghai, Guangzhou, Zhongdian, Xi'ning and Kathmandu of Nepal.
There are daily flights shuttled between Beijing, Shanghai, Guangzhou, Zhongdian, chongqing, Xi'an, Chengdu and Lhasa, especially between Chengdu and Lhasa, there are 8 flights everyday, it is the best places to enter Tibet. And the flights between Kathmandu and Lhasa are just available on every Tuesday and Saturday.
If you are going to enter Tibet from Kathmandu, you'd better to arrives in Kathmandu earlier, or you need to pay for the urgent fees for china visa, click here and you can find more information for entering tibet from Kathmandu.
You can also take the train to Lhasa, there are daily trains from Beijing, Xi'an, Xi'ning to Lhasa, and the train to Lhasa from Chengdu, Shanghai, Guangzhou, chongqing every other day, just click here and find the acurate leaving dates for each cities. the best places to enter Tibet by train is Xi'an and Xi'ning since most of the trains will pass these cities, and you will not take too much time on the trains.