How can I prevent mold stains on the bathroom ceiling?

Mold appears because of humidity and lack of ventilation, so the main thing is to get that damp air out of the bathroom - open the window after each shower or leave the exhaust fan running for a while. It also helps to clean regularly with a vinegar and water mixture or an antifungal product, and dry wet surfaces well so you don't give mold a chance to grow. If the problem is serious, consider installing a dehumidifier or improving your bathroom's ventilation, because once it appears it's tough to get rid of.