Mildew can form on your shower curtains even if you don't use the shower very often, and it's unhealthy as well as unsightly. Combined with the soap scum that naturally forms anywhere inside the shower, it can make your otherwise clean bathroom look like a high school locker room. Both chlorine bleach and ammonia kill mildew, but since these cleaners produce toxic fumes when used together, it's best to stick with bleach and leave the ammonia in the cupboard.
Put 1/2 cup of chlorine bleach and 1/4 cup of laundry detergent in the washer and start the cleaning cycle. Let the washer run for two or three minutes.
Pause the cleaning cycle and put the shower curtain in the washer along with two or three dirty towels. The towels prevent the shower curtain from crinkling.
Take the shower curtain out of the washer as soon as the spin cycle ends. Take it over to the tub and shake it out. Then hang it back in place and let it dry. Gravity and steam from the shower will straighten out the wrinkles left over from washing.
Check the washing instructions on the label and wash the curtain according to the instructions, using the hottest water allowable. Add colorfast bleach to the wash cycle.
Soak the curtain in a 1-to-1 solution of vinegar and water or in salty water after the wash cycle is complete. This helps prevent mildew from growing back.
Dry a fabric curtain in the dryer rather than simply hanging it up to dry. The heat helps control mildew.