Paint a concrete floor black and create a south facing wall with as many large windows as possible. When the heat of the sun hits the black floor through the windows, the heat will be absorbed into the concrete and gravel, and then released slowly over the course of the night.
Keep curtains open and allow the sunshine to hit as much of the floor as possible on sunny days when it's cold outside.
Close the curtains at night and on cloudy or rainy days to keep more of the heat that the floor radiates inside the house.