I'm in the process of selling a 27" Apple Cinema Display - already packaged it.
I first wrapped in in bubble wrap - an inch or so thick.
I then placed it diagonally in a large cardboard box, and filled all the void with packing chips and air bags.
This stops it moving around, and since its in the box diagonally even if something hits the outside of the box it will be very unlikely to go though to the screen.