There are two things to consider when it comes to shopping for a backpack:
1) Keep the weight of the back pack as light as possible. Ideally, the weight should be kept as low as 15% of your bodyweight.
2) Keep the weight as spread out as much as possible. In order to do this, chest, waist, and two shoulder straps are highly recommended. This will help to decrease all of the stress on the joints of the spine.