I have a Chrome Mini-Metro messenger bag that I use when out on my road or mountan bike, where I don't have my cargo rack mounted. They're a little pricey, but I find it's a lot more useful and comfortable than my old backpack when carrying just about any kind of load. One big advantage with the messenger bag is that like your backpack, your stuff stays right with you, whether on or off the bike -- but the load is distributed a little lower on your back, and your center of gravity is much lower, making you more stable and less wobbly on the bike.

They come in several different sizes... I like the "mini-metro" for daily commuter usage, but also have one of the full-sized Metropolis models for when I really have to haul some freight...