I am the science teacher in our homeschool. One of the things we learned about is the 3 states of matter; solid, liquid, and gas. An interesting thing happens to water as it gets cold. It goes from a liquid to a solid state. Chickens can't drink ice so you need to do something to keep the water warm.
One choice is to bring the water into a warm building every night and then take it back out every morning. This will work but you can't forgot to take the water out every morning. If the idea of hauling water in and out does not appeal to you, then you need to get a waterer that keeps it warm, or find a device to keep it warm. Dale did a great job talking about waterers in this article and video.
You can find waterers that plug in like the one we have in our coop. It looks like a dog bowl with a plug. I don't love it because it isn't enclosed but it is cheap and works OK.
If you already have a metal water heater you can get a heated plate that sits under the water can. I like using metal as much as possible in the coop as it inhibits mildew and other growth in the water.
There are also a lot of products that you plug in and then submerse in your water. The benefit of these is that you can find one to fit in almost any waterer that has an opening at the top. Any of these waterers or heaters should be available at your local feed store, or you can even order them on line through tractor supply company or amazon.com.
The first year we had chickens, the egg production went down to almost zero once the nights started getting cold. We didn't know what to do. Uncle Dale told us they needed to be warmer so we got a heat lamp. Within a few days of getting a lamp, we started seeing eggs in the coop. I think that heat lamps are the best things to keep your chickens warm enough so they can use some energy for egg laying. Most of the heat lamps use infrared lights. They won't bother the chickens at night. They don't need to heat the whole coop. You want to place the lamp where the chickens roost so they will be warm at night.
Good luck with your chickens. If you make sure to keep moisture out of the coop, allow for ventilation to let moisture out, use good bedding, keep water warm, and employ the use of heat lamps you should be able to get eggs all winter.
What suggestion do the rest of you have for getting the chickens ready for winter?