I'd also have to say tires; I change out tires when the temps drop, as I know the snow won't be far behind.
This year, I went with a set from "across the pond" -- DMR Redshift (26x2.25), which have an aggressive tread pattern like the Maxxis Ignitor, but seem to roll better for me. VERY LITTLE slippage in the slop.....
They've worn well, too, so when I pull them off next week, I can hang 'em and save 'em for next winter!
2nd on the list would have to be lights; strong blinky tail lights make it a lot tougher for drivers to "not see me".