Let it glow, let it glow, let it glow!!
There was al ong thread on this very subject at mbz.org. your area should have their diesel fuel optimized for the conditions in your area. Other than that, few adjustments are needed for temperatures in the teens. I live in North Central WA, and my daily driver (300D) lives outside 20's and teens). It has never failed to start. If you're not sure, plug it in, use extended and multiple glow cycles and a good "real" synthetic oil. Redline, Amsoil, Mobil or Delvac One, or Bardahls. If your car is pre- 1986, a valve adjustment in the fall in preperation for winter (required every 15,000 mi) is also a very good thing.
Chec with your local fuel distributer about their fuel, and if you're not sure, follow the owners manual for instructions on mixing kerosene.