I've found that the number one thing is that the machine can not move around inside the box. And the worse is if it's inside a case and not padded so it can't move around inside that case.
If the machine can move - it will. Even if never rough handled, just the vibration and stop/start of the truck can cause the machine to rock, and those trailers can take some nice hops going over bridge decks or other bumps in the road. I've traveled on some that could rattle your fillings out of your teeth - and that is even in a car.
The other thing - no packing peanuts or anything loose like that. the machine will vibrate it's way to the bottom, and then you'll have no packing to protect it at all. Peanuts and loose stuff like that do nothing to keep a machine from moving around.
I also like the part about removing anything that can be removed and wrapping it separately. I've got a lot of bent spool pins -