If I carry a camera bag on my shoulder, my back gets screwed up by the end of the day. I recently brought a Tamrac pack that loads from the top not the side [back]. The ones that load from the back are excellent devices to dump all the equipment on the ground.
Search Tamrac for daypacks. I got the model 750 [or 752] and I can put a Hasselblad with 50mm, 80mm and 250mm lenses, a 120 folder, a Nikon slr with 28mm - 300mm zoom and 20mm to 35mm zoom, Nikon SB-800 strobe, flip strobe bracket, film, filters and other needless crap that I shlep in hope that maybe one day I will use it.
Steve