How to adjust a backpack to fit your back
If you're going to be hiking long distances (over a mile) with a heavy backpack, it's important to fit it to your size and shape. Each brand and model has a different adjustment system, but the fit would look similar once adjusted to your back size:
1) Waist strap sits over your hips (not under and not above) and is as tight as can be, placing most of the weight of the pack on your hips.
2) The lower end of the shoulder straps reaches to about 2 inches below your armpits.
3) The upper end of the shoulder straps sits is in line with your shoulder blades.
4) The shoulders straps curve with your shoulders without putting much weight on them.
Here's a video about how a backpack should end up fitting on your back: