I make several adjustments for the winter that have already been mentioned by others. However, I think the most important tip for winterizing chickens is to leave them alone in the cold. No heaters to warm the coop.
I am all for heated water, but as long as you only raise chickens that are suitable for your area, they are fine in the cold. If you get them used to a heated coop, they can die should a storm knock the power out. Even a generator lasts for only so long. Last year I didn't lose a single chicken, even at below zero outside temperatures. Remember! Chickens generate heat, and with a closed coop, the windchill is not a factor.