Mark makes a good point about sizing the insole.
Years ago I learned this from a Red Wings Shoes store manager. The real way to measure your foot, in many cases, is from the back of the heel of the foot to the ball of the foot - NOT to the end of the toes. Many people have "short" toes. The placement of the ball of the foot is important for proper arch support and shoe flexion. It turned out that I would take from 1/2 to a full size larger than I was wearing when I walked into the store. Sure, if someone has long toes, then those must be accomodated, but the store manager gave me the impression that situation is less common than my situation.
If anyone has high arches, like I do, then an arch support may help as well. Under the constant loading/unloading of the foot with the additional weight of the pack (esp. a heavier one which might be req'd for a long trek, or anyone needing to carry heavy camera equipment, etc.), my feet would ache from the arch flattening a bit under load and then returning when unloaded - the arches act like springs and so flatten a bit with each step. This is a good thing. In a prev. post in this thread, I mentioned what I used and found to be quite satisfactory.
Also, this too can be important when sizing footwear. High arches tend to flatten a bit with each step, causing the foot to lengthen - don't want to jam the end of the big toes into the toe box with each step. Lost two big toe nails this way when I was using some La Sportiva Mountaineering boots - talk about pain with every step. Wasn't equipped to stay out in the weather which turned bad & HAD to descend. I was merely breaking in the boots in prepartation for a vacation trip. Boots were GREAT ascending, but descending was another story. Toes jammed forward with each step down. Ended up placing my feet sideways - at least the mountaineering boots kept me from "rolling" my ankle when I would step down this way. Result was the dreaded "Black Toe" (toes in my case; two to be precise) - actually mine even turned "red" from the blood - socks were soaked in blood also - prob. lost a couple of ounces of the precious red stuff. No exaggeration. It became quite "squishy" - not from sweat or water either, mind you. Blood never really clotted while descending since each step "broke" the forming clots (also the Ibuprofen I had been taking might have contributed somewhat to this "non-clotting"). Like I said, it was quite painful (like the old Scotsman says, "It's better felt, than telt") As a result, I was forced to miss the vacation trip with my co-workers. So, the moral of the story is proper shoe fitting is a must for self-preservation and oneness with the Dao of the trail/mountain.
If only I had known then about using lighter weight footwear. All of my foot problems have disappeared with light weight footwear used in conjuction with a lighter pack. [Note: Ray J mentions using trail runners even in the mtns - except for kicking steps in frozen snow, then light fabric hikers are sufficient. Too bad I hadn't read his book earlier.]
Hope this info helps someone.