Heating a Greenhouse

Back in November we finished building our greenhouse lovely, isn't it? I was super excited about all the things I could grown and overwinter because living in Northern California our winters are pretty mild.
Except this year.

I new frost was coming the next night, so I dug up my heirloom Cubanelle Peppers and Padron peppers and moved them into the greenhouse.

 I figured all would be well. I mean glass and plastic are great insulators and good enough to protect plants from frost, right? I was really, really wrong. My lovely peppers were dead and I was so upset.

After the hard freeze we had back in December was over everything in  the garden withered and gone except the thyme, parsley, rosemary, and garlic (thank God), we started researching all we could on insulating, heating and energy efficient heat sources. We found a few good ideas.

 As of now, I have a seedling mat keeping my new heirloom pepper seedlings, lettuces, lavender seedlings, borage, and dill seedlings warm enough. Next week I will be planting my tomato seeds, Mortgage Lifter and Hillbilly and maybe a few other varieties. When a freeze warning comes about I do fill a metal bucket with coals from the fireplace late in the evening and place a stone with a hole cut out on top for ventilation (my idea) in the middle of the greenhouse. The temperature in the greenhouse stays above freezing and everything seems to be growing beautifully.

Live and Learn, having a greenhouse is a lot more than throwing a bunch of plants in a transparent building and letting them be, it is a science.
After the spring planting is finished we have a few projects for the greenhouse to make it more efficient and better insulated.

How do you heat your greenhouse? Any ideas on this would be awesome since we are still learning!

Thanks
Heather