Check weather conditions when you are planning your trip. Once in China, travelers wishing to visit Tibet must join a group, which can be arranged by almost any Chinese travel agency. The travel agency arranges for the necessary permits and collects any fees. More information is available through the Chinese Embassy or one of the Chinese consulates in the United States, or, while in China, at the U.S. Embassy or nearest U.S. Consulate General. There have been some reports of robberies and assaults along remote highways near China's border with Nepal and in areas near Mt. Everest.
Most people enter Tibet through China or Nepal.
To enter through mainland China, non-Chinese nationals need a Chinese tourist visa. Currently, only tour groups are officially allowed into Tibet. When I went, a group could comprise two people. Sometimes, a group has to have a minimum of five or six people with confirmation of an organized tour itinerary through a Chinese travel agent. Some travel agents can arrange for you to enter on a group visa and then split from the group upon arriving in Lhasa.