Summarize this article in one sentence.
In addition to reducing the overall load carried, you can also decrease the likelihood of soreness or injury by making sure that your child wears the backpack properly.  The weight of the pack should be distributed primarily over the shoulders and upper back, with the chest in support — these are some of your strongest muscle groups. Straps should be snug without cutting into the shoulders.  The bag portion should be centered on the mid-back, with the bottom of the bag sagging no more than four inches below the waistline.  The bag should never touch the child's backside while being worn. Ask your child to think of the backpack as a temporary conveyance — like a delivery truck — and not as a permanent home for her stuff — like a motorhome.  She should use it to carry what she needs when she needs it to where she needs it.  Load and reload it frequently with what she needs right then.  Kids should use a locker and/or desk as their primary storage spaces, not their backpacks.  Recommend that she make more frequent trips to lockers or desks in order to unload and reload the backpack with what she needs at that moment. Procrastination can also cause overload problems, by causing a kid have to bring all her books home on Friday, for instance.  Work with her to make sure homework and assignments don't back up to the point where everything needs to be done all at once. It may be a long shot depending upon your school's budget situation, but you may want to ask whether your child can have two copies of her textbooks — one for school and one for home.  The adoption of e-textbooks (at least as a supplement for homework) may be a more practical and cost-effective solution. Make sure you educate your child about avoiding carrying a bag on one shoulder. Some kids may want to switch from a backpack that rests on both shoulders to one shoulder, such as a messenger bag. This can put stress on that shoulder, in addition to the child hiking his shoulder up to keep the backpack in place. This can lead to shoulder and neck pain if it is done on a regular basis. Both shoulder straps must be used to effectively balance the load — even if slinging the pack over only one shoulder is considered “cooler.” Most of the prevention measures for backpack safety involve strains and injuries caused by overweight loads.  However, when it comes to injuries that lead to emergency room visits, less than one-fourth involve the shoulders (12%) or back (11%), and the most common cause of significant backpack injuries is tripping over one (28%).  The majority of all ER-worthy backpack injuries involve the head or face, hands, wrists, or elbows, all likely spots for trip-and-fall injuries.  What does all this mean?  While it is important to take precautions against your child lugging around a backpack that is too heavy, make sure she is just as vigilant about paying attention to her surroundings to avoid injuries caused by tripping over a backpack or being struck by a swinging bag, strap, buckle, etc.

Summary:
Ensure a snug and secure fit. Don't carry everything all the time. Avoid carrying a backpack on one shoulder. Know the most common hazards.