Puppies, small dogs, and Seniors are especially vulnerable to extreme weather. The safest and easiest way to keep them warm is to bring them inside with you. If you are worried about potty accidents, use a crate or confine them to a small area with an easy to clean floor such as a laundry room. Fortunately, adult dogs who are accustomed to living outdoors rarely have accidents in the house because they are used to going outside.

If your dogs already live inside, some may be reluctant to go outside to potty. Small dogs and puppies especially don't like to go outside to potty in the cold. Accompany them and make sure they "do their business" outside, or you will likely have accidents to clean up. You may want to invest in training pads as a backup if you are really having trouble getting your small dog to go outside. Small dogs, especially elderly ones, appreciate sweaters, too when going out to potty and even inside. Of course, never leave them unattended with clothes on for safety.

Indoor humidifiers are good for you and your pets to alleviate dry, itchy winter skin. Just a good cheapo from your local department store is fine. A cold-mist humidifier is safer in general with children and pets (but you can't use medicated solution in it for colds), either hot or cold for $10-15. You can spend more money and get an ultrasonic cool-mist humidifier, but it's not necessary. Just make sure it has all of the modern safety precautions, especially for a hot-water one (automatic shut-off if it tips or runs out of water, etc.), and be sure you properly maintain it. The heating element or wick must be cleaned regularly to avoid mold or bacteria overgrowth, for example (see the manual of your particular model), and place it in a safe location.

Weatherproof The Outdoors

If for any reason bringing your pets inside is not possible, be sure they are in a protected small area with windbreaks/shelter and things to keep them warm. The area for sleeping in overnight should be very small so that they can stand up and turn around but not much more. Of course, this is only for sleeping overnight and helps to keep in body heat. If you have more than one pet, let them sleep together for the body heat. Lots of warm bedding should be added to the sleeping area. Straw is great because they can burrow in it and, unlike blankets, they usually won't eat it. You can get a bale of wheat straw at a feedstore and a big bag of pine shavings to mix with it, which will also help with fleas and make them smell nice. Pine shavings are less likely to cause allergic reactions in pets than cedar, but test it with your pet before putting it in their sleeping area over night to be sure. A wind flap is great on outdoor doghouses, but make sure your pet understands it and will use it confidently. Otherwise, they will sit outside because they're afraid of it.

If you do have small animals you must house outside overnight in the winter, consider investing in an under-bedding heater with proper safety precautions for chewing, etc. (not just a heating pad--they make mats designed for this purpose which are much safer!) If you use any kind of space heater in your home or kennel, be sure the pets cannot get to it and it is in good repair. Same goes for kerosene heaters, etc. Consider a humidifier to counteract the drying effects of forced air heating. It will be good for you too. Again, with proper safety precautions.

Fresh water is a must at all times! Fill the water dish up with very warm (not hot) water several times a day (or use a heater designed for his purpose, again, being careful of cords and such). Drinking cold water burns more calories, so even if it doesn't freeze, it will be better for your pets to drink warm water in the winter.

Additional food and/or higher protein food for pets spending any significant amount of time outside is a must in the winter, and feeding in the evening will help them keep warm if they must stay outside overnight. Also consider adding a daily fatty acid supplement, as this will help them build up a protective layer of fat and healthier coat. Even if they are just going outside for potty breaks, they'll appreciate the extra insulation.

Grooming for long-haired pets is extra important in winter. You do not want to shave them and take off that protection, but they need to be brushed/groomed regularly. Matted tangled hair holds in moisture and dirt, which means a matted dog who gets wet will stay wet for hours. A properly groomed, clean coat is like layers of clothes to us.

Feral Cats & Strays

If you are caring for a colony of feral cats, they probably have a place to go, but if you know where it is, make sure it has as good an enclosure as possible and lots of bedding, so they can snuggle in and keep warm together. If you want to make extra sure they have a warm bed, this is an inexpensive solution.

Buy a plastic storage tub(s) from Wal-Mart or Lowes. I call them giant tupperware.

Cut a small hole in the side, just large enough for the cats' heads to fit through so less heat leaks out.

Take the lid off, put a crate quilt down, then some straw, and snap the lid closed.

Flip the tub upside down so it is sitting on the lid and tuck into the woods or anywhere you feel you can safely tuck it away. Try to have it near something else to block wind and add extra heat. Also consider that the cat may need to exit in a flash if another animal comes after them, so do give the hole a wide berth.
Put some canned cat food inside on the side farthest away from the hole to entice them inside.
The same can be done for feral dogs, but the hole does need to be large. They cannot squeeze into smaller holes as well as cats.

Stray cats like to get up in the engine of cars--very warm for a while after turned off--and they don't leave when it cools off. Moving engine parts are highly dangerous (fanbelt injuries/fatalities are fairly common for stray cats in wintertime). Thump on the hood of your car a couple of times and/or honk your horn before cranking it in the morning. You might just save a feline life.

Be careful of antifreeze. It tastes sweet and is one of the biggest winter hazards to unattended animals. It is deadly! Be sure antifreeze is disposed of properly and in case your neighbors don't know this, watch out for it for your pets and other animals. If you see a puddle of it anywhere (it's a light green color but translucent and almost seems to glow), throw dirt into it until it's well-covered.