I usually loosen the string tension a bit.
Take off trem arms or similar. These can usually go in a Ziploc and be taped inside the box lid once the guitar is securely packed in the box.
Then pad anything which might want to move around, for example strings, a trem bridge, etc. You want to ensure nothing is going to scratch the finish, fretboard, pickups, etc.
Wrap any plastic or thin foam back around it.
Somewhere here I will put extra bubble wrap around Gibson angled headstocks. But not so much that a hard strike will snap it.
After that it is packing it in the box, so the guitar itself will not move around. Movement is the enemy over the long distance.
Was it double boxed? If so, same thing. You want to pack the inside box in the outer, such that there is no movement there either.
Then be sure you insure it for full value when you ship. I know you are supposed to have 4-6 inches of protection around items, but I send and receive guitars that do not.
That probably covers what I do. Again you do not want it moving around, so you can add bubble wrap, those brown paper things, styrofoam peanuts, whatever it takes once it is protected and in the box.
