Heat tracing cable might be an option. Tyco thermal has a site advertising their line of it.
http://www.tycothermal.com/usa/english/There is a downloadable pdf that explains the idea in a lot more detail here.
http://www.tycothermal.com/assets/Americ..._Guide_0509.pdfThey also sell a controller that turns the system on if it gets close to freezing temperatures (I think they say 40*F) but I believe most of the heating cable systems are self regulated for temperature anyhow.
Not an expert in this or related to the company but it seems more effective than a heat lamp would be.