You can make any refrigerator more efficient by making make sure it has a few inches of space behind it to keep air circulating around the condenser coils. Vacuuming your coils also can improve circulation.
Keeping your refrigerator too cold also can waste energy. The recommended temperature for refrigerators is 37 to 40 degrees (3 to 5 celcius) Fahrenheit and the recommended temperature for freezers is 5 degrees Fahrenheit (-15 celcius).
Keeping your refrigerator full also will help it retain the cold and cool more efficiently. If your fridge is sparsely occupied, you can add a few water-filled containers. On that same note, smaller refrigerators generally use less energy than larger ones. If your refrigerator is rarely full, you might want to consider replacing it with a smaller one.