Run the preheater or glow plugs, depending on your model, numerous times (3 or 4) before attempting to start...good luck. Otherwise, go and start it every few hours and allow it to warm up.
I agree. The important thing is to not let it get too cold. Besides difficulty starting, your lubricating oil will get too thick to do its job of reducing wear, possibly resulting in engine damage IF you manage to get it started.
A tank of fuel is cheaper than a damaged engine. Run it frequently overnite to keep it warm, and get a block heater as soon as possible.