I have heard of people pouring a small amount of oil into the drain--
not sure if it was cooking oil or mineral (motor) oil, or if it would
matter which. This would be especially for drains that are not that
easily accessable. Otherwise, if the drain is easy to get to, and gets
no normal use, just pour some water into it periodically. Water
evaporates- things like that are just part of maintainance.