Bring these tips to the store when looking for a back-to-school backpack for your child. (And see our backpack Ratings, available to subscribers, for models that fit your child.)

One size does not fit all. The bottom of the backpack should align with the curve of the lower back--and not more than four inches below the waistline. The shoulder-strap anchor points should also rest 1 to 2 inches below the top of the shoulders.

Inspect before you invest. Look for sloppy stitching or loose threads (which can indicate poor manufacturing); fabric edges that can fray and weaken the fabric, and possibly get stuck in the zipper; and zippers without fabric flaps covering them (which can allow water to seep in).

Comfort, convenience, and safety are important. Features to look for: wide, padded shoulder straps to distribute the weight over a large area of the shoulder; an abdominal strap to distribute the pack's weight evenly on the back, waist, and hips; and reflectors or reflective fabrics to add visibility at dusk or dawn.

Prevent injury with a lighter load. Load and wear it correctly--the American Occupational Therapy Association recommends carrying no more than 15 percent of your body weight in a backpack. The less your child carries, the better.