An annual professional inspection and service should be conducted by a trained water heater service person. Items that they should include in their service and inspection are:

Inspect and clean burner assembly (gas and oil models).

Inspect exhaust flue (gas and oil models).

Flush out sediments by draining 1-2 gallons from the tank.

Test the pressure relief valve.

Drain the expansion tank (if it is a conventional-type tank) or re-pressurize the expansion tank (if it is a diaphragm-type tank).

Ensure the fuse is tight if the circuit includes an electrical fuse.

Check for evidence of leaks or corrosion.

Check the temperature thermostat on the tank. If it is higher than 130°F you're likely wasting energy. To test the actual temperature of the water, place a meat thermometer in a path of hot running water upstairs.

If the water heater is more than 10 years old, consider replacing the anode rod. This rod is designed to protect the inside of your tank from corrosion.