Get the heavy stuff as close to your body as is possible. This last summer we saw some guys heading up The Beaten Path with heavily loaded internal frame packs. One guy had a large item clipped to the very back of his pack. As they loaded up their packs, the guy very nearly tipped over backwards!
Put heavy stuff up high and again as close to your body as possible. It sounds a bit weird, but the heavy stuff should go high up – unless you know you will be rock climbing. The higher up it goes, the more centered you can get it over your hips. Look at the natives in African countries – they carry large, heavy baskets on their heads. This puts the weight straight down on their spine, allowing them to keep their back straight and the load balanced. Others carry two large buckets on a pole across their shoulders – again, the weight is forward and mostly over their spine and hence their hips.
Balance the pack. I always seem to struggle getting the weight well balanced. If you put the tent in one side, separate the poles and put them on the other side. If you can’t find something to balance out the weight of the tent, put the tent roll and poles across the top of the pack after everything else is inside. Balance is ultra important. Nothing is worse than having a lopsided pack.
Utilize space. It is amazing at how much stuff you can get in a pack if you use your head. If you have a pot or container that will be used for cooking, fill it with something before you stuff it in. This is just plain common sense.
Leave clothing and other deformable items until last. They can be stuffed into space left in the pack after everything else is in.
Add items that you will need to get to periodically in outside pockets or clipped to your shoulder straps. Things like drinking cup, canteen, pepper spray, compass, map, etc. should be readily available so you don’t have to dig around in the pack if you are taking a 5 minute water break.
Put things that don’t get hurt by moisture (and are light weight) on the outside of the pack. I usually add my two sleeping pads (yes, I am getting old) to the top and very back of the pack. These are foam pads and if they get wet, oh well, I can wipe them off before throwing them into the tent.