If you dig down & use something heat-retaining like sand for flooring, that will help.

If you "double-pane" the sides by using 2 windows instead of 1, that will insulate just like it does in homes.
Caulk or foam insulation applied from a tube will reduce wind penetration.

When a drop in temps is due, be sure your greenhouse plants are well-mulched & huddled together as close as possible.

Then stuff some mulch or crumpled newapaper between the pots for added insulation, & water everything well.

Water is a wonderful insulator.

Here, it often gets down in the 20s in the winter time, & we keep lots of garage-sale sheets & blankets around to cover things.

Autumn leaves are wonderful protection for plants, but sheets are easier to pull off when the sun comes out!

Entire bags of autumn leaves stacked as high as you can pile them around the outside of the greenhouse will insulate, too.

Every autumn, I try to check the neighborhood on the night before trash is picked up, & bring home other people's bagged leaves:

They have many uses, from insulating individual plants to insulating the entire greenhouse,
to stuffing them in the bottom of pots for drainage,
to mulching flowerbeds,
to composting them with coffee grounds & kitchen scraps to improve the soil,
they're the most frugal things in the world (free),
& using them in my garden reduces the amount of stuff going to the landfill.

Many people here toss burlap or shade cloth over the greenhouse when the temps are due to drop;
it comes off in springtime, & in the height of summer, it goes back on to keep plants from cooking in our intense sun & heat.

If the budget runs to it, you can hang fluorescent shop lights with full spectrum or ultra-bright bulbs over your seedlings.
the most expensive part of this idea is the bulbs, but they're still cheaper than grow-lights.

Finally, you can get a small electric heater at one of the big box home improvement stores.

A friend of mine had one that was only about 10" square, but it was enough for her small greenhouse, & I think she said the price was under $30.

They have safety features now, such as automatic shut-offs in case the heater gets turned over.

Best luck, & have fun!

I'm making a greenhouse on the cheap.
I took my cues from Wyndyacre here on the GardenWeb.
So far, I've collected most of the windows I need and all the concrete blocks.
I have to scavange the bricks and wood.
After that, it's just building it thats gonna be hard work.

Here is a link that might be useful: My Greenhouse Project

Just a little idea--I 've had 2 green houses destroyed--one was homemade and it went in a bad storm. the other I purchased and a tree landed on it.

What I want now is some cold frames--enough green house already. Cold frames can be made cheaply with some wood and old windows--and do the job with alot less hassle. You can even make them to fit right over raised beds.

I just replaced two single pane sliding glass doors with new low E double pane doors so now have four large sheets of tempered glass with aluminum frames from the old doors, two each 4 foot by 6 foot eight inches and two each 5 foot by 6 foot eight inches. I was thinking of using them to either make a small green house or four huge cold frames. Any ideas?

I recently acquired 4 sliding glass doors as well. Got them through freecycle.org. Thinking of doing large cold frames. Anyone else done this with large windows/sliding glass doors?

Check out the Garden Junk forum. Search for green house. There is a beautiful one posted there recently bade from windows and it has pics of how to do it too.

Hope this helps!

~~Mimi