What red said:
1. Use a hair dryer carefully - NOT a small acetylene torch. Many people accidentally set their houses on fire that way.
2. Let water drip all night when the temps dip - hot water lines freeze faster than cold lines [someone will probably explain the physics of this] - the water bill is a small price to pay for hot water.
3. Insulate the line in the crawl space - foam pipe wrap is fairly cheap.