1. Do not let the water freeze, even for a few hours overnight. Hens that get even just a little dehydrated are much more prone to getting weak and it takes them a very long time to get rehydrated, hens just aren't that thirsty in the winter. I do like heated dogbowls. Many don’t care for them, but I find them very very helpful. I am at school all day so am unable to tend them throughout a freezing cold day. I also put a little splash of Braggs into the water, I just think it is good for them. They like the taste so drink more water.
2. Do not let your eggs freeze. If they freeze they crack a bit from the expansion. A cracked egg isn't a good idea to eat. When I find cracked eggs I try and scramble them up for the hens. Many people will disagree with this, they will tell you that your hens are more likely to eat their own eggs as a result. They are probably right, but I haven't had that experience and they seem to love scrambled eggs. I hate to waste an egg. So collect the eggs more often when it is cold.
3. Do not heat or insulate your coop too tightly. While it is true you don't want big drafts rolling through the coop a tight coop is just as bad. When chickens breath they put moisture in the air and because they spend more time in their coop in the winter there is just is more moisture in the coop in general from breathing and droppings. Moisture and humidity can create an unhealthy situation with mold and potential respiratory issues. Also a really tight coop can have the smell of ammonia to build up which can be terrible for you and the hens. A good guide is protected, well ventilated and not drafty.
4. Do not let your hens get bored. Cut up a squash, sprinkle corn cobs around, hang a cabbage, put a roost in a new spot. I don't think they need "toys" they just like scratching about for snacks.
5. Do not deprive your hens from light. Let your chickens out of the coop for a walkabout, let them out in the winter even in the snow. Don't keep them cooped up in the coop (make sure they are protected). Chickens need lots of light to continue to lay through the winter. I use lights on timers. It costs more in electric bills, however hens need light. I also have covered their outdoor run in clear corrugated plastic roofing so when they not in their nesting boxes they have access to natural light.
6. Do not forget to go over each bird to look for signs of cold distress. Do this often. I apply Vaseline to their combs in the winter. Also make sure the coop is clean: see moisture above. A good technique in the winter is the Deep Bedding Method. It is very easy to manage, you end up with compost, they birds are always entertained scratching about, and it keeps your birds healthy.