Originally Posted by gibbain
Oh, and I would have to second what other people are saying about a rack and panniers instead of a backpack. Makes a big difference.
One downside of the rack and pannier setup is that when you hit a pothole or need to go up or down a curb, they might come off and the straps and hooks get caught up in your gears or spokes. I had this happen once too often and switched to a pack designed for cyclists, eg. Vaude. As long as you keep it lightweight, it works fine.