Cis4 left out the most obvious option:
It sounds like your work might not be very physical? If not, it can help to find some desk exercises or something the last 15 minutes to increase your metabolism. I've found as long as I'm dressed appropriately for the weather and I haven't been completely sedentary, the car is comfortable from the beginning because it get's me out of the wind and weather. Then as the car heats up, and if the drive is more than 5 or 6 minutes, I just take off my winter jacket.
You might want to read this article about the wisdom (or lack of it) of letting your car idle when cold:
http://www.businessinsider.com/here...-doing-to-your-engine-and-its-not-good-2016-1
(credit to Brillo54 who posted this in another thread)
edit: I've found a brisk manual clearing of any snow/ice also warms me up a bit from the inside-out. It's much more satisfying kind of warmth than having warm air blow on cold skin. Also, make sure you have had enough water to drink before you leave work. Being a little dehydrated can make it difficult for your body to generate any warmth of it's own.