Insulate your walls. You can use bubble wrap in the winter, and weather sealing. Also, if you choose a greenhouse will thicker walls, it will retain the heat much better. For example 3.5mm polycarbonate will be less insulating than 10mm.
There are several options, in greenhouse heating. You may choose electric, Liquid Propane, Natural Gas, etc. Of course, there are several points of concern, the biggest of which is safety. Be sure that the heater that you choose is safe for use in a greenhouse. Also keep in mind that combusted gas, from wood, oil, kerosene, or gas, contains components that are harmful to plants. Ventilation is essential. (There is one exception to this rule. -Non venting heaters. That is, heaters that operate on a high level of burning efficiency, and exhaust mainly CO2, which is great for plants.)
You may also want to consider a Heat Thermostat. All you have to do is plug it in, and plug your heater into it. This will help you to maintain a constant temperature.
How To Calculate Heat Requirements The following formula can be used to determine the approximate heating requirements of your greenhouse: A X D X 1.1 = Btu’s.
“A” is the total wall and roof surface area of your greenhouse. “D” is the difference between coldest outdoor winter temperature and the night temperature desired in your greenhouse.
"Btu’s" is the heat requirement. Subtract 30% if greenhouse is insulated using double glazing or polyethylene liner. Subtract another 30% if it is a lean-to greenhouse on heated wall.
A creative way to maintain heat in your greenhouse is by the use of water. You can use a black drum filled with water, positioned on the North side of the greenhouse. It will radiate a good deal of heat.
Another important thing to consider is soil temperature. If you can keep the soil at 65 degrees F your plants will tolerate cooler temperatures, much better. Heat mats work well.