Drape a water-heater blanket - really nothing more than an overcoat - around your tank to keep heat from leaking away.
Also, wrap pipe insulation around the first five feet of both the hot-water outlet pipe and the cold-water pipe leading into the tank. You can find the stuff at any hardware or plumbing-supply store.
Taking these measures will knock as much as 15% off the unit's operating costs.
NEXT: Call for service