I've shipped bikes a couple times, and trials bikes are easy to ship - they're smaller, burlier, fewer parts, and probably already dented up anyways.
a basic rule is just to reinforce anything that can be bent easily. On roadbikes, I'd take the chainrings off and zip tie em to one of the wheels (like a disc rotor) so that it's a lot harder to take a hit. for any wheel that i take off, i put an axle in the dropouts with nuts on each side of the drop out and tighten it. otherwise it's just a big lever waiting to get bent.
put loose parts/bolts in a bag that can be closed and pack it separately or attach it to your bike securely. while you're at it, tie/ziptie/tape/attach everything on your bike together - your box can get pretty f**ked up, and holes will appear that bolts/pedals/headset spacers will fall through.