To freshen drains (not unclog them) and help keep them free-flowing, mix 1/2 cup baking soda with 1/4 cup table salt.
Pour the mixture down the drain, followed by 1 cup heated vinegar.
It will foam and bubble. Let it stand for 15 minutes — longer does not work any better.
Follow by running hot tap water for at least 15 to 30 seconds. This is especially good for seldom-used drains, like in a guest bathroom.