Near as I can tell there is nothing that can be done to get your mileage as good in the winter as it is in the summer. There are a couple things you can do to help. One thing I learned from my Vette's real time tire pressure sensors is that the cold weather lowers the tire pressure. This is do to the fact that the air is more dense at lower temps in accordance with the formula PV=NRT. Pressure is directly proportional to temperature (measured on a absolute scale). 32F is about 273K and 86F is about 303K making the tire pressure 90% of the summer pressure at freezing temps. Check em and air em up!

Safety note: If you are maxing out your pressure when it is cold you will overpressure your tires if it gets warm. Please be careful.

The other thing you could do borders on cheating but it would help to keep the car or at least the engine warm. Warm up time is greater in the winter, reducing MPGs. You really wouldn't save any net energy by heating the car as it would take energy to do the heating but your numbers would be higher.

There is also such a thing as a warm air mod I've heard the others speak of.

I've been dreaming about a dual intake system that would pre-heat the intake air in the winter. One intake would be just like the stock intake but the other would be double wrapped around the exaust, using the exaust heat to warm the intake air in the winter. As far as I know that is my own idea and no one actually has one for sale. Maybe you could be the first to fabricate such a system?