Many helmets have removable liners that can be washed if they start to get smelly. Wash your helmet liner by hand in dish soap or laundry detergent for delicate fabrics and lay it flat to dry on a towel. Keep your helmet out of the direct sun in the back of your car, and out of hot environments. Store your helmet in a cool, dry place in the microfiber helmet bag it came with, or in a clean cloth bag. Do your best not to.