The grill unit should be cleaned out after every 5 barbecues to keep the airflow regulated at the correct level. When completely cool, simply remove the grill and use remove all grey ash.
The removable grill top, once completely cool, can be removed from the grill unit and cleaned vigorously with a wire brush. Try to do this as often as you can in order to keep your grill unit in the pristine condition you received it in.