you want to fit the iMac in a box and not have the head of it move all over the place. Make sure that the iMac neck is down to its lowest point and then bring it up a little bit and pack it well so that it is impossible for it to go down... you might use some rolled towels.. or rolls of paper towel
wrap some other towels (the bath or beach type) around neck and try to make sure that it can't move up when the box is closed... if it moves a little bit, it probably won't damage the computer, but you don't want to have a stress on the neck.
Another important thing.. don't use anything that can fit into the vents on the computer. Use books, newspapers, and towels to help build around the computer.
Finally, it might be a good idea to put a tea towel over the monitor to help protect it.
Good luck with the move!