Not only skirting but a vapor barrier on the ground beneath the house. Makes the biggest difference.

And I agree with Peace. I keep the thermostat at 64 degrees. When I sit on the recliner in the evening I use a lap robe and a cat for coziness. I rarely turn on the actual heat. This house has an oil-fired water pipe system with radiators so there is always some hot water moving somewhere. Plus I'm on the top floor and I get a lot of the residual heat from below.