One of the reasons my race bike gets so few miles.. I hate backpacks and the hot spot on one's back.. And most of my cycling is used for errands or commuting.. My tour and commute bikes have panniers. My race bike does not.. So , I take the bike with panniers and a trunk bag..
For the few days, I take my race bike and I know I'll bring something lite home. Like a newspaper stop. I try to just take one of those cloth backpacks that are light and breath..And can be inserted in my jersey pocket until the time I need it.