* Insulate the roof and at least the north wall, and perhaps the east and west walls.

* Use double glazing, whether you you glaze your greenhouse with glass or polycarbonate.

* Provide thermal storage. Water is the most practical way to store heat. Most low cost solar greenhouses use barrels or oil drums, filled with water, for heat storage.

* Make sure your thermal storage is in direct sun for as long as possible

* Paint the insides of your greenhouse white, to increase light reflection onto your plants. Paint your thermal storage black to absorb more heat.

* Orient your greenhouse so that its longest wall faces south. Tilt the south wall so that it faces the Winter sun.