You need to look at quite a few various models of manufacturer's backpacks to locate the correct one. It should be comfy when empty, but also when full. Changes should be made to all of the straps in order for a good fit.
Straps, Belts and Lifters:
There are lots of items to change the fit of a backpack to get it feeling comfy:
Hip Belt: Make sure the hip belt is sitting over the hip bones. A good hip belt will be suitably cushioned as well as adjustable. Some hip belts may be (once the pack is purchased) form suited for your body.
Shoulder Straps: These really should be sitting on the middle of your shoulders. Not really too near towards neck (or perhaps this may contribute to strain. Not way too far out on the arms or the shoulders won't be carrying sufficient of the weight.
Sternum Strap: This is a strap which is fastened to the shoulder straps at chest level and connects the two shoulder straps together. It helps by holding the shoulder straps in place while moving around a lot.
Load Lifters: These are straps which join to your shoulder straps and farther up the pack to help position the weight upon your shoulders.
Stabilizer Straps: These are located on the hip belt and help move the weight toward the body to keep it in place.
You can change all of these items to maximize the comfort level of the outdoor backpack. You will also choose to make changes when on the trail.