In reply to sikcby: Fit is much more important than padding, the comfiest sack I have has no padding on the hip belt, minimal padding on the straps/back and no adjustment. But it fits me really well.
I used to have a 100l military style bag and it was hellish to carry, mainly because I'm quite narrow in the shoulders and it was obviously designed for someone bigger. Try and get one that fits your body type well, I use a macpac for heavier loads now and it's not too bad. Most bigger sacks have good back adjustments or different lengths but make sure the shoulder straps are in the right place for you. If you buy a bag where the shoulders sit wide it'll put the weight on the outside of your shoulder, if it's too narrow for you it will cut into your neck.
In terms of carrying heavy loads properly, unfortunately 50lb plus is at best going to be uncomfortable after a few hours. Keep the heaviest items central and close to your back. Most people find getting the weight sitting on their hips helps. I find making adjustments as I walk helps to distribute the pain throughout the day making things a bit more bearable. Stuffing your hat or gloves under the shoulders sometimes gives a bit of variation.
If you're expecting chaffing (i.e. a multi day trip, heavy load, hot weather and just a base layer on) then tape up before hand and save youself the misery of having your skin exfoliated.
The best thing you can do is buy a smaller bag which will force you to take less. The heavier your load is the less you need to add to make it worse, if that makes sense. For example the difference between 20lb and 30lb isn't great but the difference between 50lb and 60lb is. The best way is always going to be to try and reduce your load, even if it's just a little but if you really need to take a lot of weight (for e.g. carrying coal to a bothy) unfortunately it's just going to be a suffer fest!
Ben