According to The About Health page "Right and Wrong Ways to Wear a Backpack":
Wearing a backpack wrong can lead to more pressure on the neck, shoulders and back, according to researchers.
Always wear both shoulder straps rather than slinging it with one strap on one shoulder. It takes a moment longer but can help prevent problems.
Adjust the shoulder straps so the backpack is high on the back and the shoulder straps are comfortable on the shoulders.
The backpack should not sway from side to side as you walk.