Keeping chickens warm

We often have bitterly cold temperatures around here during winter. How do you guys keep your chickens warm and healthy during the winter months? This is my first year with a flock of chickens, so I could use some tips.
I live in California, so it doesn't get cold enough to be a problem. My sister raises chickens in Iowa and she puts some petroleum jelly on their combs and wattles before they go to roost in the evening. She says it protects against frostbite.


I'm a Southern boy now, but I used to be a Midwesterner. We used the deep litter method to help heat up the coop, though it was a pain to clean each spring. It's important to make sure there's good ventilation without letting a bunch of wind blow throw the coop.


We also live in an area that doesn't experience extreme temperatures. However, chickens are hardy birds, but the hens can stop producing eggs if they aren't kept warm enough. Heat lamps can come in handy, except that there is the risk of fires starting.