One way to prevent mold and mildew in your bathroom is to have a properly working exhaust fan to pull that moist air out of the room and send it outside. Manually controlling these fans via a switch can become a hassle.
You can purchase exhaust fans that will automatically come on when the humidity in the room reaches a certain point. These particular fans will stay on for 30 minutes or so after they are activated before automatically shutting off.
Another alternative is to install a timed/un-timed switch to control your exhaust fan. After hitting the timed switch the fan will turn off after a set period of time. Using the un-timed switch will of course stay on until you turn it off.
Example Humidity Sensing Fan: Humidity Sensing Fan
Example Timed Switch: Timed Switch