When you know your cat or dog is ready to sleep for awhile, place a rubber hot water bottle under the bedding, but close enough so they feel the heat. I use these myself sometimes during the day and definitely at night. They have never leaked. When heating the water, remove just before the boiling point.
We have an old Bassett Hound with a crooked paw and arthritis. Since she does not bite at anything I put a heating pad, on low, and place it in various places. She loves it. Even though she has been through three other dogs she is still the queen!
By
12/15/2010
I voted this tip down, not because the idea isn't good. It's a very good idea. But I've used hot water bottles on myself many times through the years, and they get cold after not too awfully long. I use a different way of keeping my dogs' bed warm. I have an electric "throw" blanket. I put it on the floor underneath my dogs' bed and turn it on low. Then I put a small lightweight blanket on top. I have the ones they sell at Wal Mart for $6 right now. One is plenty for my 2 dachshunds.
The electric blanket keeps the bed at just the right temperature for them on a cold night, and doesn't cool off over time. It only costs pennies a day to keep it on. And the blanket on top holds the warmth in so they aren't warm on their bellies and cold on their backs. My babies love it. An added advantage: They aren't sneaking up into my warm bed after I go to sleep! (Not that they aren't welcome, but they love their bed).