Never carry more than 15% of your body weight in your backpack. If you weigh 100 pounds you shouldn't wear a loaded backpack heavier than 15 pounds.
Load heaviest items closest to the back. Pack items neatly, and keep them organized to keep books and materials from sliding around in the pack and shifting the weight.
Wearing a Backpack
Always wear both shoulder straps to distribute weight evenly. Wearing a pack slung over one shoulder can cause a person to lean to one side and curve the spine.
Adjust the shoulder straps so that the pack fits snugly to your back. A pack that hangs loosely from the back can pull you backward and strain the muscles between the shoulders.
The bottom of the pack should rest in the curve of the lower back. It should never rest more than four inches below the waistline.