Ditto on the thermostatically controlled heat tape, but consider also installing a FreezeAlarm or similar device. The FreezeAlarm is available through the NH Electric Coop (
http://www.nhec.com/products_freezealarms.php). Having it gives a lot of peace of mind, knowing that it will call you if there is a problem with temperature dropping too low inside the house, and that you can call into it at any time to query current temperature and alarm setpoints. You might even choose to drop the heat tapes if you have the alarm installed, unless of course there are some cold spots perhaps subject to freezing while the rest of the house is ok.
Does the house have a full basement, or at least an enclosed and insulated crawl space? House configuration is all important in deciding what protection to have.
The subject of protection/monitoring for infrequently used vacation homes has come up often on this site. A quick search will call them up.