West Coast Imaging does an excellent job of packaging. For smaller prints (I think 20x24" and smaller), they put an interleaving sheet on top of the print, put the whole thing in an archival plastic sleeve, tape it to a sheet of corrugated cardboard with a border around it, and package so that there are two sheets of corrugated cardboard on each side of the print, and the whole thing goes into another corrugated cardboard box. If there are negs, transparencies, or disks, they sleeve them and tape them between two of the other sheets of cardboard. The package is very sturdy.
For 11x14's (unmounted prints or smaller prints mounted to 11x14") I've found some nice sturdy mailing boxes they sell at Staples.