Wear the backpack on the strongest of your back muscles!
Pay close attention to the way the backpack is positioned on the back. It should rest evenly in the middle of the back. Shoulder straps should be adjusted to allow the person using the backpack to put on and take off the pack without difficulty and allow free movement of the arms. Straps should not be too loose, and the backpack should not extend below the low back.
Keep the size of the backpack within reasonable proportion to the body using the pack. This will allow appropriate functionality for the user of the backpack, allowing them to maneuver properly.