Last time I was in the hardware store I noticed electrical outlet sealers on display with the weather stripping and other insulation supplies. It had never occurred to me until then, but electrical outlets could be a big source of heat loss (especially in older homes) because they are basically a hole in the wall (as you can see in the picture).
That makes electrical outlets a great candidate for insulation. So I picked up a 14 pack of FrostKing electric outlet sealers (these are what I am holding in the picture) for $2.17. The pack included 8 outlet sealers, 4 switch sealers, and 2 other sealers.
Installation of these couldn't be easier:
The outlet insulators that I bought said that they are made from "fire retardant plastic foam" you might want to look for that when you buy them.