Does anyone use a greenhouse?

I do fairly good with my little 9x12 but it can get so terribly hot in there.

We use a small radiator type heater in the winter which usually keeps it warm enough at night but I think the temps fluctuate too much.

The greenhouse usually runs 20* warmer than the outside temp.

It is not set in full sun but does get the late evening sun.

Got any tips to help keep the same temp?

What is the best temp to try and keep it at? 80*?