Cold Start?

I have had trouble starting my 1979 300D every winter. I only have trouble now when it gets in single digits as today it was 9F. My car just simply doesn't have enough boost to start it. I have been using diesel kleen, but don't understand how a lot of people get their diesels started in -20 weather when I can't get it started in just 9F. Let me know what I should do. Also how do you keep the door locks from freezing up on the outside of the car???