Measure your back for the correct rucksack frame size

The attachments didn't display properly on my computer, but the basic advice is well-taken. Second only to properly fitting shoes or boots, a properly sized and fitted backpack is one of THE most important decisions you'll make in preparing for a walk like the Camino. Any outdoor store worth its salt can assist with this, and before you go into the store you can Google "fitting a backpack" or "fitting a rucksack" to familiarize yourself with the procedure. Don't forget to size the hip belt properly as well -- be sure there is enough play in the adjustment portion of the belt to keep it tight as you inevitably loose weight on any long trek (see note below). Pay attention to the procedures for adjusting the pack after you lift it onto your back, and adjust all the straps in the correct sequence: 1) hip belt, 2) shoulder straps, 3) load lifters, 4) sternum strap.

Load and wear the pack on your training walks to make sure it actually fits and feels well in actual use. If not, exchange it for one that does. Verify that the place you buy it will accept returns and exchanges within a reasonable time limit; if not, go somewhere else. It's not worth it to save $10 or $20 if you'll lose the entire cost on an ill-fitting pack -- or worse yet, spoil your Camino because of back pain caused by a poorly sized and fitted pack.

Finally, take a look at the photo in my avatar. It looks like my pack should have had a longer torso length, but in actuality, my hip belt was too small and I ran out of adjustment length on the buckle strap. This caused the hip belt to slip down over my hip bones as I lost weight, which in turn caused too much of the weight to be borne by my shoulders. If you have to hook your thumbs under the shoulder straps to relieve pain or pressure on your shoulders, the pack is not properly fitted (even though it was when I started).

It's a good point about measuring yourself or rather having a trained staff member do it.

I'm not that tall but as it turns out I have a longer torso. As a result I was able to get a better fitting backpack.

As an addendum to my earlier post, most pilgrims try to buy a backpack with a capacity of 30 to 40 litres, which many consider to be a large day pack or overnight pack. The videos and fitting diagrams from most manufacturers suggest a 45-degree angle between the top of the shoulder and the point where the load levelers attach to the main body or frame of the backpack. That is normal for a 50-liter or larger pack simply because the pack is physically longer than the 30 to 40-liter backpack used by most Camino pilgrims. The overall dimensions of such a smaller pack may not permit the optimum placement of the attachment points for the shoulder straps, or more especially, the load leveler straps. The buyer should be aware of this and take that into consideration. Life is a series of trade-offs and compromises .