Cover leftovers.
Break out the shrink wrap (or better yet use reusable containers!), and keep your food items covered when you place them in the fridge. The moisture in uncovered foods and liquids evaporates in the dry air of the refrigerator making the compressor have to work even hard (and run even longer). New refrigerators are about 10 times more energy efficient that their ancestors but the fridge is still one of your homes biggest energy hogs. Anything you can do to ease the power your fridge uses makes a big impact.