Don't cool your fridge more than you need to! Set the fridge at 5°C and the freezer around -15°C (you can check the temperature reliably by leaving a regular thermometer in there for a few minutes).
Make sure there is room for air to circulate around the outside of the fridge so it works efficiently. Position it well away from the oven and any walls that radiate heat.
Also try to keep the fridge door closed as much as possible, as letting air in forces the fridge to work harder!