1. Pull your fridge or freezer from the wall and Vacuum the condenser coils at least once a year. When the condenser coils are dirty they can't release the heat from the fridge into the air so they don't work as well.
2. Check your gasket. This is easy just take a dollar bill and close it in the door with part of it sticking out. if you can easily slide the dollar out they your gasket isn't keeping in the cold air and needs replacing. Also make sure you clean any dirt out of the little grooves on the gasket to help prevent wear and tear.
3. If your fridge is really really old then it may be time to consider buying a new one. Look at those Sunday sales ads and decide what you're looking for before you head out to the store. This will help you get exactly what you want. Keep in mind all those extra features may just suck down more electricity so decide before hand if you really need them.
4 Defrost the Freezer. If you don't have a frost-free freezer and you have more than a half inch of freezer frost then it's time for the Great Thaw. Put all your food in a cooler the fuller the better and unplug your freezer. You can help speed things up with a couple pans of hot water. Never use a knife or other tool to remove ice you can damage the inside of your freezer.
Another "small" thing you can do: when you are putting away your groceries after a trip to the store, gather all your items for the fridge in one place. Then you can open the refrigerator one time and put everything inside.
Now that it's clean and defrosted, keep your freezer efficient by filling empty spaces with old milk jugs filled with water (remember to leave an inch at the top for expansion). These jugs are great for camping trips or picnics to keep things cold without the water mess of ice cubes.