I prefer messengers or panniers but sometimes need a bag. This is my trial and error learnings. Put the heavy things at the back of the pack, so it is on your back. And keep straps tight. If you can get the bag to ride right above the small of your back, it's best. Or at least it is for me! (Disclosure I use a medium sized pack... this one. It's not huge and fits up high.) My 'real' bag, which is big and low, was really annoying!