The word was last year you had to go with a tour group, be part of them, going via china. once you are there, you can break off from the group, but going by yourself is tough and even impossible I believe. a buddy of mine takes tours in there if you are interested. he works for a big company. unfortunately, that is all i know!
--- "I have nothing to say of my working life, only that a tie is a noose, and inverted though it is, it will hang a man nonetheless if he is not careful." -Yann Martel
Yes, i have been in China recently, not in Tibet, but i know that you can go indepently there.
You can fly from Chengdu or Kunming, thatīs the easiest and fastest way to go.
Or you can also go overlanding in a long long 4X4 way through the mountains till Lhasa. (7 days way)
Thatīs only interesting by the experience cause the price is almost the same than a flight ticket.
I donīt know in Chengdu but in Kunming you can buy easily a plane ticket to Lhasa ... look at some ""travel agency"" at Beijing Lu street near the KunHu Fandian Hotel.
Haven't being to Tibet in the last 6 mos. but being to Tiben in 2001 last time, and as they said above, you'll end up paying large amounts of money in brives and "cuotas". Chinese policemen do not care about you as a tourist, specialy in Tibet, actually, they don't want you there ! , so they will make the less effort posible to help.
You need permits from one deprtment to the next, and they are not marked like in other countries, suddenly you have a check point and hey stop you and ask for the documents.
Your best option will be to book a trip with a local travel agency either in Kathmandu or Chengdu, and stay over. Your "expedition"-tour will be, lets say 7 days, and you can stay over and book your return for the 15/th day, or any time posible you please to ! .......
There are many tours available from kathmandu. You can get travel into Tibet and then just meet lift home a week or so after. Be careful about your visa though. Some companies will only supply a group visa with the cost and this could cause you problems.