I have used a Colman RV propane heater, ...then moving my plants later and later out in spring, hardly needed any heat, I just use a electric base board heater, put double plastic up for more insulation.

I'm a long ways from moving anything out yet, but in the spring when I start using my greenhouse, I keep a 5 gallon pail of water in each corner to help keep the temp from dropping too low overnight. I've done pop can solar heaters as well, but they sometimes get too hot through the day. It'll keep the greenhouse 5-10 degrees higher than outside overnight.

Here is a link that might be useful: pop can solar heater instructions

I've never done it but I've heard of people using the solar blankets that you use for a swimming pool.

My green house it too large to heat with an electrical heater but I have used a propane tank with a heater attachment along with the wood heater when we have had lows of under minus twenty