All the directions I've read state that the center of gravity of the pack should be slightly above your body's center of gravity and as close as possible to your back.

This means that the pack needs to be loaded according to your build. For most people, that depends on your gender. My own center of gravity, as a somewhat hippy female, is pretty low, at or just below my waist. For most men, the center of gravity is several inches above the waist.

I have an additional problem when packing because the only one of my items that is heavier than any other single item and therefore should be at the pack's center of gravity is my food. By the last few days of the trip, though, this is no longer true. As a result, I have rather an issue balancing my pack the last day or two. I probably should put a rock in the food bag!

During the day, the two items I want accessible from the pack interior are my insulating clothing (my rain gear and windshirt are already in an outside pocket) and my first aid kit (I never have needed the latter during the day, but if I should need it, there would be no time to go rummaging through the pack). Those two things are on top. Like everyone else, I pack my sleeping bag in the bottom of the pack. My food is on top of that, generally horizontal, putting it just above my waist (most men would want it higher). The first thing I want when I stop for the night is my shelter, which I normally carry in one of the outside side pockets so I don't have to open the pack if it's raining. The alternative would be on the very top of the pack. After the shelter is up, I unload my entire pack inside the shelter, so it doesn't really matter in what order I pack it.

I most definitely agree with Glenn's last paragraph!