What to do:

Install heat reflectors between radiators and exterior walls. Radiators emit heat on both sides of the radiator, thus the side facing the wall does a really good job of heating the exterior wall and not such a good job of heating the air in the room. Installing heat reflectors behind the radiators can reflect more heat back into the room, reducing the amount that escapes through the exterior walls.

You can make inexpensive reflectors by wrapping pieces of cardboard with aluminum foil and slipping these homemade reflectors between the radiator and the wall. You can substitute a car windshield heat reflector for the aluminum foil for an even more effective reflector.

Savings:

The amount saved will depend on the number of radiators in your home, how well insulated the exterior walls are and how leaky your exterior walls are. Savings can range up to 10% of the total cost of heating according to studies in Britain, but average savings would probably be closer to 5%. That translates to $70 for the average household this heating season.