Seal cracks in doors, windows and foundations. Small openings concentrate cold air, creating a cold blow torch that can quickly freeze pipes.
Open cabinets and closets that contain water lines to allow warm air to circulate around the plumbing.
Allow cold water to drip through faucets. Running or dripping water can help prevent water lines from freezing; however, even running water can freeze in sub-zero temperatures.
Consider installing water pipe insulation. Local building supply stores carry a number of products designed specifically for this purpose. If you decide to use a heat tape, be sure it has the UL seal of approval.
Check pipe insulation periodically to make sure that it is dry. Wet insulation can be worse than no insulation.
Make sure that outside spigots are turned off and garden hoses are disconnected.
If you suspect that your plumbing is not properly protected from freezing, hire a licensed plumber to perform an inspection.