What you need to keep your greenhouse warm in the winter months:
55-gallon water barrels, painted black
Large wood burning stove
Bricks or stones
Greenhouse's collect and trap heat, but in the winter you sometimes need to provide additional heat. Especially in very cold months. Properly orienting a greenhouse structure can help with your auxiliary heat.
The best thing to do for your greenhouse is to fashion it in a east to west position. You want one full length side to be exposed to the southern sky as much as possible. This will use the winter sun to warm your greenhouse. You want to have the back of the greenhouse north to a existing structure like your home or a outbuilding. This gives your greenhouse extra wind protection.
Another option is to use water to store heat. A 55 gallon water barrel in black placed inside the greenhouse will capture the sun's heat. Any heat accumulated during the daytime will release at night, after the temps have dropped.
Its not a bad idea to insulate your greenhouse either. For plastic greenhouse's insulate with a foam sheet. These sheets get placed over your greenhouse at night, and then removed during the day. Installing some extra plastic to the interior will also add additional insulation. For hard structures, double glazed glass is a great greenhouse cover. Insulate your end walls and north wall with a moisture proof foam insulation.
You can also install a wood burning stove, placed in the center of your greenhouse. Making sure the vent goes through the roof, with a metal plate to dissipate any heat. This ensures your plastic will not melt. Large circulation fans near your roof of your greenhouse will help to circulate warm air. You can also stack bricks and stones around your wood stove to create thermal heat.