What's your favorite way for saving electricity or gas with minimal impact on quality of life?

Here’s one of mine (November-March):

Two sets of ice packs. One is outside the house during frosty days and nights and the other inside the refrigerator on the top shelf (cold air is heavier). They get exchanged regularly. Works also with tap-water-filled plastic bottles.

I wonder if half a billion refrigerator owners did this, how much money would they save per year and how many tons of CO2 would not be released into our atmosphere?