I will work outdside if it's above 0*F, but often stay home until it gets to at least 10*. Then it's somewhat manageable, depending on conditions... wind, snow, etc.
I can't see working outside when it's near 0 or below. How many days are like that for you?
Anyway, what I do when it gets real cold is let the engine warm up so the defrost is blowing warm air, and I use a torch to warm the glass from the outside also... takes both to get the w/s warm and keep it warm, unless the chip is down in the defrost area.
I also can use one of my shade/shield things to block the wind if needed, to keep my fingers and nose from falling off.
Also, if you can't get it inside... at least get it out of the wind if possible, behind a building, etc.
Keep you tools inside a warm car as much as possible too, while you work.
When bringing a car inside, I like to let the defrost run for a while first and get the w/s warm. Otherwise, you pull it in and the glass is ice cold and foggy... and you still have to warm it etc. If the car runs a while first, the w/s stays warm and dry, and there is some heat under the hood too.
I have been in this business "part time" for three years. I really enjoy it and have to say the topic on this board are great. I am a member with ACT as a billing source, but am billing more and more on my own. My concern is weather. I am in Alaska and run into cold weather/out door situations. I use garages whenever possible, but don't really know my limits. Any suggestions. 
I hate the winter it's behind me now I'm enjoying my 60-degree days.