I started out splitting my commute between days, so I would carry the following day's clothes in the car. Then I started commuting the whole round trip in a single day, with a backpack, but that sucked because of the sweat and just general fatigue with the backpack on my back for 15+ miles. So finally I just rigged up my road bike (my primary commuter) with a rear rack and trunk bag, with fold-out panniers. Ahhh much better!

If you don't normally commute on your plastic bike, but want to use it on nice days, I would plan ahead with weather forecasts and such. Take the clothes for that day on a prior day, and then just don't carry anything extra. Lightweight mode. Sometimes I leave my trunk bag at work if I know I'm driving the next day, and commute home in "lightweight mode" with nothing on the rear rack. It can definitely make for a faster ride.