Disconnect all outdoor hoses which allow water to drain from the connected pipes. When a hose is attached, one overnight freeze can cause the faucet or pipes to crack.
Seal all cracks and holes with caulk or foam sealant, and seal or cover any access doors that can go unused during the cold spell. You can cover doors with plastic and hay or straw bales work well against crawlspace doors.
Measure the diameter of any pipes at risk of freezing. Buy matching-sized foam pipe insulation from a home improvement store or hardware store. This insulation is split down the middle and you just slip it over the pipes. You may need to cut the insulation to fit certain lengths.
Wrap heat tape instead of foam around pipes that may be especially susceptible to freezing, such as those in completely unheated areas. Plug the tape into an approved electrical outlet when the weather is predicted to turn cold enough for the pipes to freeze.
Add heat with a small electric heater controlled with a thermostat, or, in small areas, add a light bulb screwed into an attachment plug base to provide enough heat to prevent freezing.
Open cupboard doors under all sinks in the house allowing heat to reach water pipes in crawlspaces and other connected areas.
If temperatures drop very low and you cannot adequately protect the pipes, leave faucets trickling to keep water flowing through the pipes. During this type of weather, turn on faucets full force every two or three hours to check for any slowdown, possibly indicating a freezing pipe.
If a plumbing pipe does freeze, thaw it with a hair dryer on a low setting. Thawing the pipe too quickly can cause it to break.
Close the whole house water shut-off valve if you plan to leave for an extended period during very cold weather.
Consider adding a permanent heat source to a crawlspace problem area if you live in a climate with harsh winters. An opening in a warm air supply duct can prevent freezing pipes, and the opening can be capped off when the weather is not so extreme.