When it gets really cold, I'll open the cabinet doors under the kitchen sink to make sure that some warmer air can circulate in. Some people recommend letting a small stream of water run to prevent the line from freezing.

I once lived in an apartment and had a roommate that would put a space heater under the sink... I worried that was a fire hazard though.

If you're ultra worried, shut off the water to your place and let the water drain out through the lowest spigot...