I keep a complete set of clean clothes in a drawer in my office. Also, I choose a bike with fenders if it's raining or I think it's going to rain. I pack anything that needs to stay dry in a plastic bag inside a pannier, to make sure it will stay dry. If it's night, I have a light. If it's cold, I have a military surplus balaclava and wool skiing tights, wool socks, and lots of layers. The only thing that stops me is significant snow or ice (which doesn't happen much), and I may get studded tires for this winter.

Mike