Water heaters should be flushed out at least once a year. The drain valve is not at the very bottom of the tank so you need to use the cold water inlet valve to get the water moving off the bottom of the tank. The dip tube takes the clod water to the bottom of the tank so opening the water supply valve while flushing helps a lot. I alway drain and flush a gas heater once a year. When there is lots of water left in the tank (from just a quick flushing) the inlet pressure is not as great on the tank bottom as when the tank is emptied so more build-up will remain.
if you have never made a mistake, you probably haven't done much.