The previous post on shipping 2d artwork linked to an in depth tutorial. Shipping 3d and unusually shaped artwork often presents even more challenges than shipping 2d work.
the 2015 National Council on Education for the Ceramic Arts Conference has made a video available of a workshop on packing 3d artwork . Much of the video discusses various methods of packing in wooden crates. However, it also points out a number of tips for proper packing of artwork that can be applied to artwork packed in (less expensive) cardboard boxes as well.
It's a long video but well worth the listen. I found the last speaker who is a gallerist particularly helpful in regards to what galleries do and do not like in regards to receiving, unpackinng and re-packing artwork.