An excellent topic! ....and no two situations are exactly the same, however, a few important points and design goals of the packaging can be carried through to many situations.
In general, the packaging must not only protect against being crushed, but must protect against rapid deceleration forces (such as the box being dropped or sliding within a truck. ) and, in the case of a complete sign or art piece, must protect against the "heavy" parts (base, transformer, etc) from acting as a hammer against the fragile parts. (glass, or even the faces or substrates used)
Bare tubing by itself does well when fastened to a piece of cardboard as this keeps everything dimensionally stable--vibration or impact forces cannot cause part of the tubing to move more than any other part. Make sure that sheet is larger in all directions than the unit, then float it inside of an oversize box using peanuts or excelsior. Then place this box inside of another larger heavy wall box that will protect against crushing. Use eggcrate foam between them, or something similar. In very large pieces, this outermost box should be a crate, perhaps with reinforcements on edges and across large surfaces. Shipping companies tend to stack things and they do not always care what warnings you put on the box!
For a complete sign or artpiece, sometimes it is best to pack tubing separate from bases and transformers. When this is not possible, the following approaches are helpful:
Make sure the frame/substrate that the tubing is attached to does NOT flex! It needs to provide a rigid and dimensionally stable attachment for the tubing so that attached masses (transformers) cannot cause it to flex and break the tubing--no matter which direction the acceleration force comes from. If this is not possible, best to remove the transformers or the tubing and have them reassembled at the destination.
Build a rigid "cage" or enclosure over the tubing that is attached to the frame/substrate--this prevents the entire assembly from moving and exerting force upon the tubing when it contacts the wall of the container. (prevents the mass of the sign assembly from acting like a hammer against the tubing)
This entire cage/enclosure can then be floated in an oversized box similar to how bare tubing on a sheet can be.
Then this inner carton can be floated with the foam within the outer crate or heavy wall box.
This gives pretty good protection by preventing the assembly from being able to shift and contact its container (prevents the hammer effect) and also provides for cushioning to dampen the acceleration forces from impacts, while the outermost container prevents crushing.
Nothing is foolproof and other methods can work too, but whatever you go with, the goals should include crush, flex, and deceleration protections.
One item I have not touched upon but which you may want to consider is that even a ceramic shell electrode can damage itself in shipping if the vibrations happen to hit a resonant sweet spot and make the shell flex on its lead-in wires and have the ceramic collar hammer on the glass envelope from inside. That's a hard one to prevent, and I've even had brand new electrodes arrive broken because of this......all I can say is the multiple padding layers help prevent it.
Lastly, if you have a specific thing you care trying to ship and don't mind sharing a photo or two, it may help in devising a more tailored strategy.