How can I insulate my fireplace when it's not in use?
Similar to @BMitch's answer, I used a magnetic cover to cover the vent below my fireplace.  We found this was the primary cool air leak for our fireplace and I bet it is yours too.  It works great, and is very simple to remove before starting a fire.  Since it is black like the fireplace, it is not even noticed.  I bought one from this site selling the exact size I needed and in black. 




Here is my fireplace with it on and removed.