For 4 years, I commuted to work 20+ miles (RT) 10 months a year and carried a 8lb.+ backpack which included my laptop. Now, that I am no longer working, I freed up my back by buying 2 Nashbar townie bags to carry my groceries. They attach to each side of my rear rack. They can be detached when not needed. I can haul a fair amount of groceries with them.
http://www.nashbar.com/profile.cfm?...rand=&sku=8641&storetype=&estoreid=&pagename=
http://www.nashbar.com/profile.cfm?...rand=&sku=8641&storetype=&estoreid=&pagename=