Watch this video from This Old House to learn how to flush your water heater.
1. Turn off electricity and water to the water heater; allow heater to cool.
2. Drain water from the heater's tank using a garden hose and pump.
3. Catch drained water in a bucket for inspection.
4. If water is filled with sediment, refill heater and drain again.
5. Continue to fill and drain heater as often as necessary until the water runs clear.
6. Disconnect the hose and pump.
7. Turn on the electricity and water to the water heater.