Water freezing inside a pipe or faucet is a common cause of leaks in the winter. Though it may be counterintuitive, a leak won't necessarily be at the point in the pipe where the water freezes. Instead, it is usually be downstream of that, between the frozen section and a closed faucet. It's the build-up of water pressure in the pipe that causes the leak, not the lateral expansion of the ice. Consequently, if you're worried about an outdoor faucet freezing, take steps to insulate the pipe that supplies it, as well as to protect the faucet itself.
Cover all exposed pipes leading to the faucet with foam pipe insulation. It comes pre-slit, and all you have to do is cut it to length with a knife, open the slit, fit the insulation around the pipe and close it again. It's a good idea to reinforce the insulation by wrapping it with PVC pipe tape.
Remove the garden hose from the faucet and cover it with a polystyrene faucet cover or with rags if you're expecting one or two nights of moderately cold weather and you plan to be home.
Let the faucet drip about five drops per minute if you're expecting temperatures below 28 degrees Fahrenheit. A small drip keeps water flowing just enough to prevent it from freezing.
Turn off the water to the faucet, open it and leave it open to protect it for a long period of exposure to freezing temperatures. This is the best way to protect the faucet if you leave for an extended period.