All foreign visitors to Tibet need one or
more permits. The basic one is the Tibet Tourism Bureau (TTB) permit,
which can be issued to you by Chinese travel agencies that handle trips
to Tibet, or (if overseas and arriving via Nepal) by the Chinese embassy
in Kathmandu on proof of purchasing a package tour (there is no way
around this). If you buy an expensive package tour, the TTB permit will
only cost you US$6, but if you just want train/plane tickets, the travel
agency will inflate their cut accordingly and you'll need to fork out up
to US$50-70. For land crossings (including the train), you'll get a
physical permit that will be checked; for plane tickets, the permit may
just be an annotation on your ticket record.
Some parts of Tibet also require an Aliens' Travel Permit (ATP), which is
issued by the Public Security Bureau (PSB) in major Tibetan cities like
Lhasa, Xigatse and Ali. The list of regions that require ATPs changes
constantly, so enquire locally. Lhasa's PSB has a poor reputation, while
Xigatse and Ali are said to issue permits without any unnecessary
difficulties. If your papers are in order, the permit can be issued in
several hours for Y100.
Finally, some remote areas also require a military permit. These are only
available in Lhasa, where processing takes several days, and are only
granted for an appropriate reason.
You can fly to Lhasa and also Nyingchi but flying in from a much lower
altitude city puts you at high risk of altitude sickness because of the
quick transition. If you are in Sichuan or nearby (and aren't satisfied
visiting the many ethnically Tibetan areas to the east of the Tibetan
Autonomous Region) flying from Chengdu is the easiest option.
According to signs (posted in train stations among other places) to go to
Tibet after June 26th you need a great deal of official paperwork.
Hostels in Chengdu can handle that paperwork for a very reasonable fee.
Just show up at a large hostel and ask to book a flight to Lhasa,
they'll handle the rest (cost 2000 RMB).
The Qinghai-Tibet (Qingzang) Railway from Golmud to Lhasa started
operating in July 2006. The journey all the way from Beijing takes just
under 48 hours, costing 389 yuan in the cheapest hard seat class and
1262 yuan for a soft sleeper. Direct trains to Lhasa originate in
Beijing, Xining, Lanzhou, Shanghai, Guangzhou, Chongqing and Chengdu.
Here is a link to a Timetable and Ticket Prices for Trains to Tibet 
There are four roads into Tibet, roughly corresponding to the cardinal
North: The road from Golmud (Ch:Ge'ermu) is the easiest legal land route
at present. The landscape is beautiful but difficult to appreciate after
the long rough ride.
It's possible to travel this way by hitch-hiking on trucks if you are well
prepared (camping equipment, food and water for a day). Expect to spend
a few days. There are police checkpoints on the way but the only one
that is a problem is the one 30 km or so out of Golmud. If you walk
around it and a few km beyond you should be able to get a ride without
too much of a problem. There are plenty of places to eat on the way but
be prepared to get stuck in the middle of nowhere. There are also are
places to sleep ranging from truck stop brothels to comfortable hotels,
however these should be avoided as you're likely to get picked up by the
East: There is no legal way to travel this road (except as part of an
expensive organised tour; see Overland to Tibet) and the security is
tighter than from the north. Travellers do get through this way, but for
people who are obviously not northeast Asians it's difficult.
West: From Kashgar (Ch:Kashi) much of the way is technically off limits.
However there is a steady stream of hardy travelers coming this way,
usually hitching rides on trucks. The road is totally unpaved for over a
thousand kilometers with villages and water few and far between. The
main advantages of this way is that it passes by Mount Kailash and
through a beautiful, very remote region inhabited by nomads. You should
be very well prepared to travel this way and take everything you would
need for independent trekking: camping equipment suitable for freezing
temperatures even in summer, a good tent and at least a few days of food
(there are a few truck-stop places on the way but not always when you
want them). Expect the trip to take two weeks or more. From Kashgar it's
much farther to go to Lhasa via Urumqi and Golmud but the better
transport (trains and good paved highways) make it no more time
consuming to travel this way. There are many interesting things for the
tourist to see on the way and it is worth considering traveling this way
instead of via Mount Kailash.
South: From Nepal the international border makes any sort of breaking of
the rules impossible, so the only option is to book a tour with a travel
agent in Kathmandu. In addition, as of 2007, you need a group visa for
China itself to cross the border into Tibet, so don't bother applying
before you get to Kathmandu. The drive from Kathmandu to Lhasa takes
five days and is very rough, but pretty.
Southeast: After 44 years of closure, the Nathu La pass to Sikkim, India —
a part of the historic Silk Road — opened again in July 2006. At time of
writing, the border is not yet open to foreign tourists, but this is
expected to change some time in the future and there are plans for a
Gangtok-Lhasa bus service.