I use a messenger bag or a small back pack for carrying personal items such as spare clothing, pump,spare tubes and tools and few other things. I would never leave any of these personal tems on my bike when I lock it outside. My bikes are also fitted with front and rear racks which I use for heavy bulky items and when doing grocery shopping. People who whine and complain about sweaty back from wearing backpacks are just exaggerating the problem and making it sound as if it's some great hardship or life threatning event. There are backpacks out there made especially for cycling such as Camelbacks and Deuter and they are very comfortable to wear.