Met these guys who were on the road to Lhasa. They were pilgrims, and had been walking for months. They said they still had a few weeks to go. The entire time, they would bow and prostrate themselves on the ground touching their forehead each time. You can tell by the marks on their head -- or the creativeness of the fellow on the right.
Video of footage I took '5 Days in Tibet' http://vimeo.com/21786941