How to book a train ticket Beijing to Lhasa in advance

I want to book in advance Train ticket from Beijing to Lhasa (Soft or Hard sleeper) on the 9th or 10th of July 2012 and I want to be sure to get it. Is the booking, reservation reliable or is there a high chance that we don't get it due to overbooking or shortage of seats? What is the best way to have a save reservation?