My dog is a 10-year-old whose history includes time in animal shelters, and in various homes. He has arthritis, a progressive condition that will cause him more pain as he grows older. Direct heat through my electric blanket or heating pad is clearly very comforting for him–if there's a heated throw of some kind around, he'll find it and snuggle up. But the best solution I've found for both of us is this gently heated dog bed, which warms up to your dog's body temperature when they sit down on it.
You don't have to worry about it heating up too hot, or about remembering to switch it on or off. When your pup needs it, they get warmth. When they step away, the device shifts to "off." I bought mine a few months ago, and it is the center of his world. The color is attractive, and works with the neutral tones in my modest, minimalist living space. Strongly recommended if you have a smaller, older dog, or a cat who likes extra body warmth.
From the manufacturer's specs:
• Heater uses only 6 watts and is MET Listed
• Removable cushion and removable heater make washing a breeze
• Thermostatically controlled to warm to your pet's normal body temperature when in use
• 5-inch thick bedding keeps pet comfortable
• Removable, machine washable cover for easy maintenance
• Safe plush heated pillow insert for winter warmth
• Available in 2 sizes and 2 colors; 1-year warranty
amazon: K&H Manufacturing Thermo-Snuggly Sleeper
[Editor's note: There are two sizes: a medium and a large. Both are currently on sale, but the medium's the one I bought. It's more than 30% off today, Jan. 5, 2016.]