Two ways:
1) when abnormally cold weather is approaching, leave a trickle of water flowing from a faucet that this pipe serves.
2) Insulation. You don't really want wrap the pipe with insulation -- that will slow down how long it takes the pipe to freeze, but won't stop it from freezing. You want insulation on 3 sides to keep the cold from the pipe, but you want the heat from the living areas reaching the pipe.