when i first started commuting, i used a rack and trunk bag, but i was never crazy about having the extra weight on the bike, so i went out and bought a very spendy, but amazing, backpack with a suspended mesh back panel for ventilation and all kinds of straps and clips to get the pack cinched down just right. i absolutely love it and it means that i don't need to worry about having a rack on my bike. any of my bikes can be commuters now that i roll exclusively with a back pack.