Clean your basement thoroughly. Remove any linens, like draperies made from cotton or wool, and rugs or carpet. Mildew feeds off these items. Clean dust from the floor joists and keep the floors and walls swept and free from spider webs and dust. If you already suspect mildew in your basement, mix bleach into your cleaning water when you clean your basement. Using 20 percent bleach and 80 percent water will help to kill any mildew.
Remove the moisture from the air with a dehumidifier. A dehumidifier takes moisture out of the air in your basement. Mildew needs moisture to grow and basements, because they are underground, are usually high humidity locations.
Install an exhaust fan if you have basement windows. An exhaust fan forces air to circulate out of the basement. As the moist air from the basement leaves, it forces dryer air from outside to enter into the basement. Only use the exhaust fan in situations where the humidity outside is less than inside.