Backpacks can become very uncomfortable very quickly if not packed and fitted correctly; this article explains the basics of how to pack your backpack for comfort.
The key is weight distribution no matter what type of pack you have. You want to keep the weight as close to your body as possible and in an area that doesn't cause excess strain on your back and shoulders. This centers the weight with your natural center of gravity and transfers the weight down your spine to your hips allowing your hip / waist belt to transfer the weight to your strongest muscles, your butt and upper legs! So…
Place your heaviest items as close to your back as possible and in between your shoulder blades and above. If you're trekking rocky or more difficult terrain, try not to place heavy items above your shoulders as this could throw you off balance.
Next, radiating outward, place your medium weight items and then finally your light weight items. (Click to see an image)
I usually start by placing my lightest items at the bottom. Things like sleeping bag and light clothing etc. Also things you may need during the day and when you reach camp.
Next, put some of your medium weight items that you won't require right away and build up to where your heaviest items will go.
Then place the heavy items in and pack light and medium weight items around them to the extremities of the pack.
When everything is in, make sure you tighten all of the compression straps to secure the load. Again this helps keep the weight close to your body.