Because I commute 18 miles, each way, I put my 4 to 8 pounds of stuff in my mini-panniers and on the rack. It keeps the weight and heat off my shoulders and back. I don't dislike backpacks or messenger bags. I've carried both, but prefer having my back free to breath.

On the rare occasion that I need to bring more stuff, I throw it into a small day-pack. It's the same type pack I used for 15+ years of commuting by skateboard.