I bought some of those foam-like insulators that you just take the faceplate off the wall and put those behind them.
You mean like the little foam things you put over the outlets? That'll help a little.
If you're really serious about saving energy (and keeping the bills down) this winter, you should be looking for the big losses - uninsulated attic door, cheap windows, drafty door seals, uninsulated or poorly insulated walls and ceilings. There's plenty of places to find heat loss. Do some internet searches on heat loss and look for the biggest bang for the buck. Often, a cheap solution makes a significant difference.
When I bought this house, the garage ceiling (garage under the bedrooms) was not insulated, the AC/Furnace unit in the garage was blowing a ton of air out through some serious gaps where the plenum attached to the furnace, the wall between the garage and the basement had no drywall on the garage side (and since it was a 9 foot wall and the basement drywall was only 8 feet, had a gap above where the insulation was just flapping freely). The front door gasket was so bad that I could see light around the most of the door, and the thermal glass on the back door was missing the outside layer. The attic access had NO insulation, and no seal.
My highest summer AC bill this year (and it was a hot year here) was less than the previous owner's monthly budget electric bill. I didn't see their gas bills, but I'd bet that my heating bills are much lower as well.
It took some effort to get there, but I made it a better house in terms of energy usage, and I'm not done yet!