Proper Backpack fitting (mmdchiropractic.ca) as per the blog from MMD Chiropractic Health Centre, Hamilton, Ontario.
Wear Backpack High on Back to Prevent Pain
Always wear both shoulder straps rather than slinging it on one shoulder. 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. If a waist strap is available, it should be used, as it transfers the weight of the pack to the hips rather than having all of the weight riding on the shoulders.
Wrong Way to Wear a Backpack - Using One Strap - Sling Method
Wearing a backpack slung over one shoulder puts all of the pressure on that shoulder. Regular carrying of a backpack this way may result in bad posture habits and strain the shoulder and neck. Take the time to use both straps and position the backpack high on the back, where it won't sway with each step.
Wrong Way to Wear a Backpack - Low on Back
Wearing a backpack low on the back increases the pressure on the shoulders. This position may lead to shoulder and lower back strain. Loosening the shoulder straps makes it easier to put the backpack on and to take it off, but you should adjust the straps immediately, so it rides high on the back. This low back position may lead to a forward lean, which places pressure on the lower back. Loose straps also allow the pack to sway back and forth when you walk, which can be uncomfortable and may result in the straps chafing your shoulders.