To flush the Sediment out of the water heater just hook a garden hose to the spigot on the water heater drain and then let the water drain completely out of the water heater and then with the hot water side turned off turn the cold water side on to help force the sediment out of the water heater and let it run this way for 30 minutes or until most of the sediment comes out.
You can help extend the life of a water heater by flushing the sediment out of the water heater on a yearly basis or even every 6 months if you want too.