1.) Turn off electric power or gas before doing anything. Damage will result if element comes on when tank is dry. Turn of water supplying HWT. Note that a time switch is NOT a safe place to turn off the electricity! Do it from the circuit breaker, or pull the fuse.

2.) Drain the water heater (HWT). After HWT is drained, and hose is still attached, open and close the inlet valve a few times to help flush the sediment out. Do this 'till the water comes out clear. You may have to dismantle the valve, if there are large chunks of scale coming loose.

3.) Remove the sacrificial anode, which looks like a plug in the top of the HWT. Inspect; it should be almost as long as the water heater. Replace if any portion of it is thinner than about 1/4";.

4.) With anode out, shine flashlight inside of tank to inspect for rust. If you see a lot of rust, it's probably time to replace it...before it fails. Water heaters are normally glass- or ceramic-lined to prevent corrosion; this is also what the anode's for. The heat of the water hastens corrosion, once it starts.

5.) Open up the element access panels. Disconnect one wire from each of the elements. With a volt-ohm-meter, check to see that both elements are still functional (the resistance across the terminals should be ??? ohms, but if your meter peaks out with exceptionally high ohms, it's time to replace the element).