Crammed with everything from lunches to laptops, overloaded backpacks can give kids stiff necks, sore shoulders and aching backs. But parents can help prevent these possible pains by teaching their children a few simple rules as they head back to school.

"Parents are often surprised at how much their child's backpack actually weighs," said Dr. Lourdes de la Cruz, a pediatrician with the Sutter Delta Medical Group. "As a general rule, to prevent injury, your child's full backpack should weigh no more than 15 percent of his body weight."

How a student wears his backpack is often as important as its overall weight.

"Kids often wear their bag slung over one shoulder or so low that it sits below their waist," added de la Cruz. "This could cause neck and muscle spasms, lower back pain or even numbness and tingling in a student's arms."

While some experts disagree about whether heavy backpacks are the source of back pain in children, most agree that using good judgment when wearing one will reduce the potential risk of backpack-related injuries.

Here are five tips to lighten the strain on your child's back:

Buy a backpack with wide, padded straps to minimize pressure on the shoulders and collarbone. Avoid a leather bag - it's fashionable, but it's heavier than nylon. Carry the pack on both shoulders to spread the weight; tighten straps so the bag lies about two inches above the waist. Bend both knees instead of leaning over when hoisting a heavy bag. Consider a backpack with wheels.