Personality
When possible, choose a backpack that reflects your child's personality.
Size
Choose one that is appropriate for their body size, that is, one that sits between their shoulders and hips.
Pockets
Go for a backpack that carries many pockets, because you can use them to help even out the weight.
Padding
A lot of padding in the shoulder straps as well as in the back will help to cushion their bodies.
Double Zippers
Choose one that comes with double zippers so that if one zipper is broken, the other will be available to keep the backpack closed.
Strong
Choose one that is strong enough to carry your child's school supplies, which should be about 8 to 10 percent of their body weight.
Link to an article
The Beauty of the Backpack
http://ezinearticles.com/?The-Beauty-of-the-Backpack&id=355839