I like my "messenger backpack", a Crumpler Famous Wine Bar, for when I don't want to mess with panniers. Distributes the load well for when you're on a bike, big capacity, and has an optional waist strap that's really optional. (It comes off completely, not dangling at your sides and catching on stuff.) So that's a thing to consider if you want two straps to bear a heavier load more evenly (or for whatever reason). They only started making messenger backpacks in the last few years; before that I used Chrome messenger bags because they were one of the few reasonable, really waterproof options out there.