I have a small backpack that is good for carrying around up to about 10 pounds of stuff; but when its heavily loaded I find it somewhat lacking. The weight pulls on my shoulders and presses uncomfortably on my vertebrae. In order to keep the center of mass over my feet I need to hunch over.
I could start bringing by internal frame backpack, complete with padded waist belt and sternum strap, whenever I anticipate carrying a heavy load, but I think I have a better solution:
In normal use, it looks just like a regular rucksack, but it is actually two separate bags held together with zippers. When you need to carry a heavy load, separate the bags and divide the weight evenly between them.
Theres one other part: a yoke, which is a curved bar designed to be carried behind the neck and rest on the shoulders. The two bags hang from the ends of the yoke. Their center of mass is positioned directly over your spine, so it is more comfortable than backpack.
The yoke is constructed of lightweight aluminum and folds up so it can be stowed away when in backpack mode.
The major limitation is that you must be able to divide your cargo into two equal portions. In most cases this is easily overcome. For instance, if you go to the library to checkout the complete works of Shakespeare, you should also pick up another equally weighty tome. (Or just rip the Shakespeare in half.)