Just to follow up on what Duck on Wheels said, before I got my panniers I used a backpack. Easily the most comfortable backpack I have ever owned (and I love my backpacks like some women love shoes) is an Obus Forme. The company makes back supports for chairs, etc, but ventured into packs a few years ago.

The website is just the company name dot com, and the models I use are older and not listed (it's the one the demo guy is wearing), but they are so adjustable and keep the weight high and close to your back. They have a special padded sleeve for a laptop, all sorts of stash pockets, are expandable...

I was at a safety conference a while ago, and they were doing demonstrations with other school-type packs - the same books in each bag, and you tried each bag on for "feel". I couldn't believe how much lighter the Obus Forme felt with the exact same load. I bought one on the spot, and then saw a larger one on sale years later and bought it, too.