However, even if you aren't thinking of upgrading your fridge there are some things you can do to increase it's efficiency:
1) Check if the door seals. Stick a piece of paper between the door and fridge body and see how hard it is to pull out. If it slips out easily, cold air is probably doing the same thing! A little silicon spray may improve the situation, or look into replacement seals.
2) Clean the coils on the back of your fridge twice a year. Dust builds up on these coils and adds and insulating layer thus making the heat transfer from the coils to the surrounding environment less efficient. Just unplug the fridge and vacuum if you notice dust building up.
3) If your fridge is in direct sunlight, near a heating vent or range/oven, find a cooler place for it to be. The warmer the fridge's environment, the harder it has to work to keep things inside cool.
4) Be sure there are a few inches of free space between the condenser coils, sides of the fridge and the wall (this way the warm air has somewhere to go!).
5) Keep the freezer defrosted - a centimeter is too much!
6) Set your fridge thermostat to realistic levels.
7) Keeping the freezer full is a good thing, but the same is not true for the fridge part. Leave some room for air to circulate.
8) Quick in, quick out! This one is common sense - the longer your fridge door is open, the harder your fridge has to work!