I've been using a backpack quite a bit. You just get used to it.

I have one that has a fair amount of padding in the back. But, I also try to pack flat stuff closest to the back. There is nothing worse than having an awkward load that pokes one in the middle of the back.

I generally don't use a belt for the small soft packs, but do keep the straps reasonably tight.

Panniers are nice too.