I read (or heard) somewhere that to conserve energy, you should keep your freezer filled; if not with food, then bottled water. This helps keep the temperature cold, and when you open the door, less cold air will be lost. Additionally, if there's a power outage, you can use the bottled water to maintain the cool temperature in the fridge.
So I took some bottles from my recycling bin, washed them out, filled them with tap and put them in the fridge. By the next morning, I had a tip of my own:
Don't use glass bottles.