Get a Ranco or Johnson temperature controller. Essentially, this unit replaces the thermostat on your fridge, which isn't very precise. If you do a search on Ranco, you will see lots of posts come up -- they are a great controller.
If your fridge is going to be in a cold environment (temperature drops below freezing) then you may have to get a two-stage controller (more expensive) and have it control both a cooling device (i.e. fridge) and a heating device (e.g., heat bulb or space heater). My beer fridge is in the garage, so I cool in the summer and heat in the winter.
Otherwise, just get a single-stage controller. Should cost about $30 - $100 bucks, depending on the brand and type.