#1 Make sure the box is at least a few cm bigger
on all sides and both ends than the gigbag with the bass in it.
#2 Make sure the box is
sturdy and not flexible, baggy, or made up of smaller boxes poorly taped together.
#3 Wrap the entire gigbag (with the bass in it) in about three or four layers of bubble wrap. Make sure that includes over the bottom and top. If you don't have bubble wrap, you can use foam "peanuts" or other foam products. Make sure that the bottom of the bass and the headstock are securely held away from the ends of the box, and cannot shift to the end through the foam peanuts if the box is shaken.
#4 Make sure that whatever packing method you used, the bass cannot shift around in the box. Pick the box up and shake it. If you hear anything thumping around in there, open it back up and put some more packing materials in.
I've shipped about 20 basses this way, and they all made it safely wherever they were going.
You do not need to loosen the strings unless you are shipping from one extreme climate to another extreme climate.