The folks at Living the Good Life shared a video discussing 10 ways to heat your greenhouse. One big tip is to not plant right up next to the wall. Leave some space. 
- Use plastic bottles filled with water left sitting in the sun. Dark bottles hold heat longer and don’t allow gunk to grow in them. You can minimize the gunk (algae and water mold) by adding bleach. If you have a large greenhouse, black 55 gallon drums placed along the north side work well.
- Make a composting area using manure and kitchen scraps mixed with leaves and shredded paper. While the composting process is taking place, heat is released. Some greenhouse gardeners in other countries keep animals and plants together in their houses, you just need to watch where you step. Don’t plant directly in the composting material unless there is 6-8” of soil on top of the working mixture.
- Terra cotta (AKA Clay) pots stacked with tea lights will add warmth.
- Heat lamps can be used but you will want to add a fan to circulate the air to keep them from over heating and the closest plants from parching.
- Electric space heaters can also be used, but need to be away from wetness. Get one with a thermostat so you can control heating temperature.
- Ceramic space heater tile uses less electricity and is safe to touch; may still need a fan to spread heat around greenhouse
- Propane space heaters give off CO and needs to have an outside vent; you can boil large pots of water and leave them in the greenhouse overnight
- Geothermal heating elements are probably the most complicated. They require water circulation underground to transfer heating and cooling.
- Solar panels can be set up to create the electricity needed to run a heater.
- Wood stove will give off the most heat for little cost, but you must have wood available
Sue Sanderson of Thompson-Morgan has additional suggestions. 
- Insulate with horticultural bubble wrap. The larger bubbles let in more light.
- Use a thermostat on any heater so you only heat when necessary
- Choose the right temperature – 45ºF to 55ºF is adequate to keep young plants growing.
- Use a thermometer to record maximum and minimum temperatures to work more efficiently
- Position heaters carefully – not so close as to dry out plants foliage
- Only heat the area you need. If you are not using the entire area make a cluster grouping and bubble wrap around them to make a partition
- Use horticultural fleece on extra cold nights; don’t forget to take it off in the morning
- Ventilation is important to keep air moving to prevent diseases; open vents during warm days, close before the sun goes down
- Use a seed germinator heating pad to stimulate seed sprouting and keep young seedlings warm
Then there is the Chinese greenhouse. This passive greenhouse style has three walls of brick, clay or earth that make up the north, east and west sides. Only the south side, covered with a transparent material, allows the sunshine in. The walls capture solar energy during the day into the thermal mass walls, which release it during the night as heat. At sunset, an insulating sheet, usually made of stray, pressed grass or canvas, can be rolled out over the plastic to further slow heat loss. These features keep the temperature inside up to 45degrees higher than the outdoors. 
Billie Nicholson, Editor