If you've got an extra chest freezer out in the the garage or you just have your regular freezer in the kitchen you want to keep them both full, here's why: each time you open your freezer, cold air escapes and warm air flows in. The majority of the energy your freezer uses goes towards cooling down the air that comes in when you open the freezer door. When your freezer is full, there is less room for warmer air to take up, and the items that are in there help to cool down any air that does sneak in. So keeping it full means less energy used.
So what if you don't have enough food to fill it up? There's some tricks you can use to keep it full: scrunch up newspaper and fill it up, fill empty milk jugs with water and put in the freezer, fill plastic containers or ziploc bags with water and put in the freezer, use leftover styrofoam or bags of styrofoam peanuts. Don't go overboard and stuff your freezer to the gills to try make it more efficient, just fill enough so everything fits comfortably and you don't have to jam the door shut, if you overfill you actually defeat the whole purpose.