Wash laundry in cold water and run full loads
Added
Set your washer to cold for everyday loads and wait until you have a full load. Washers use about the same energy whatever the load size, and heating the water is most of the cost. Switch to warm or hot only for oily stains or when someone is sick.
The result
Result: ENERGY STAR: water heating is about 90% of the energy a clothes washer uses; going from hot to warm can cut energy use in half, and cold cuts it more.
Source
ENERGY STAR: Clothes Washers savings tips (opens the original source in a new tab). Check it for the latest numbers before you act.