Install a block heater. It will not only mean easier starts, but also will save your starter and battery, making both last longer. Save electricity by using a timer that turns on the block heater a couple of hours you start the engine. Use proper fuel and enough of it. Park your vehicles with the tanks at least three-quarters full.
Moisture is much less likely to accumulate in nearly full tanks.
Make sure motor oil viscosity is correct. This is important in any engine, but it is especially critical in diesel engines used in cold weather. Make any necessary repairs. If your diesel has been a bit difficult to start in warm weather, just think what will happen when cold weather arrives. Get vehicles repaired before temperatures plunge.