I have a variety of perfectly good regular backpack and messenger bags, and each I use for different purposes. One I use for sports, another for the office etc. So rather than offload and reload a bag on the bicycle I thought it might be good to find a way to simply switch bags for the purpose.
Maybe you can consider seatpost mounted racks like the Rixen Kaul Vario Rack, Xootr Crossrack or, what I have, the Basil Baseasy rack.
With the Baseasy rack, (rated for 5kgs and mounted on my seatpost), I bought one bracket for the bike but two racks (on the cheap for $3.50 each). I've tied on the rack to my bag permanently in such a way that it does not interfere when wearing it. There's a quick release for easy removal and hence easy switching.
Last edited by Shahmatt; 04-02-14 at 09:23 PM.