What to do: Give plastic shower curtains and liners a spin in the washing machine with your regular detergent and a few old towels, which help scrub away soap scum and mildew. Rehang to dry.
For shower doors, make a paste by adding a few drops of distilled white vinegar to a cup of baking soda; apply it directly to the door (it's nice and thick, so it will stick). Let sit for an hour, then rub with a microfiber cloth. Rinse and buff dry with a fresh, dry microfiber cloth.
The tub is less of an issue—a weekly scrubbing is usually enough. But for extra gleam, fill it with hot water, then drain. Apply a bathroom cleaner and let sit for 15 minutes before scrubbing.
To maintain it, wipe condensation from all surfaces after showering, and leave the window open for one hour a day to lower the room's humidity level.
Why: Those germs from your shower head (and your body) can linger in your tub.