Don't let moisture build up and cause mildew, which leads to mold! There are more than 100,000 species of mold on the planet, and one of their favorite places to live is your bathroom. Make sure to dry off surfaces that collect standing water, and also keep the room well ventilated. Run your ceiling vent fan after every shower. Install a switch timer to make sure it runs at least 20 minutes to vent moist, mildew-attracting air outside.
Also see How to Install a Bathroom Vent Fan
and How to Properly Vent a Bathroom