Wear both straps
Use of one strap causes one side of the body to bear the weight of the backpack. By wearing two shoulder straps, the weight of the backpack is better distributed.

Wear the backpack over the strongest mid-back muscles
Pay close attention to the way the backpack is positioned on the back. It should rest evenly in the middle of the back. Shoulder straps should be adjusted to allow the child to put on and take off the backpack without difficulty and allow free movement of the arms. Straps should not be too loose, and the backpack should not extend below the low back.

Lighten the load
Keep the load at 10-15% or less of the child's body weight. Carry only those items that are required for the day. Organize the contents of the backpack by placing the heaviest items closest to the back. Some students have two sets of books so as not to have to carry the heavy books to and from school.


How a Physical Therapist Can Help
A physical therapist can help you choose a proper backpack and fit it specifically to your child. Children come in all shapes and sizes, and some have physical limitations that require special adaptations.

Additionally, a physical therapist can help improve posture problems, correct muscle imbalances, and treat pain that can result from improper backpack use. Physical therapists can also design individualized fitness programs to help children get strong and stay strong—and carry their own loads!

2013 American Physical Therapy Association

Sleeping--
Get into a neutral position, either on your back (use one pillow) or side (use one pillow). Avoid lying on your stomach if you have neck pain. When lying on your side, make sure your head is positioned such that you are looking ahead, not down at your feet. Contour pillows are good to use (though they take some time to get used to), or you can use pillows that incorporate a cervical roll. 
Driving--
Stay upright with your head against the headrest and straighten arms out to keep shoulders back, Tilt rearview mirror a bit toward the roof so that the upright posture is necessary to maintain the total field of vision in the mirror. 
Using a computer--
Screen should be positioned at eye level, with head back and chin in. Sit back in chair and use a lumbar roll. Use attached arm on monitor or other device to hold documents at eye level. 
Reading--
Don't read in bed. Often, too much head flexion is created if pillows are bunched up behind the head. Instead, sit with your back against the headboard or in a chair next to the bed. 
Enjoying Hobbies--
For counter- or lap-based hobbies such as quilting, sewing, collecting items in albums, etc., don't continuously look down for long period of time without taking a break. Change your physical position and stretch in the opposite direction about every 20 minutes. 
Gardening--
Don't do marathon gardening sessions. Take frequent breaks. Half kneel or squat when working.
Cleaning--
When possible, half kneel or squat to keep spine in a neutral position. 
Watching television--
Sit up straight in the chair or on your couch. Don't slouch! Get up periodically and walk around. 

source: clevelandclinic.org 

The three natural curves of the spine plus the discs in the spine help absorb the "shock" or "forces" of everyday life. Jobs or tasks that require bending forward all day (in addition to the 3,000 and 5,000 times a day we ordinarily bend forward) decrease the natural curves of the spine, resulting in more stress being placed on the back side of ligament and joints of cervical and lumbar spine. 

• Overall bad posture results in more wear and tear of the spinal structures.
• After age 30, all the years of straining the normal tissue from slouching eventually decreases structural stability, allowing for greater wearing down of ligaments.
• Postural misalignment can cause radiating pain away from the back and spine due, for instance, to a pinched nerve; and muscle spasms can create headaches and neck/scapular pain. 
• Pain often begins as a soreness of stiffness in the neck and shoulder blades and can progress to headache pressure and band-like symptoms around head and/or back tightness that can progress to the hip, leg, and toes. 
• Many factors contribute to the onset of pain, including job duties, hobbies, household responsibilities, studying, and activities. 

info from clevelandclinic.org