Sweat stains are a part of normal life. After being in a very warm wedding last weekend, I can tell you that sometimes they cannot be avoided. If you are using bleach in your wash, scrub the sweat stains with water and lemon juice. Launder as usual. If you are not using bleach, scrub the stains with white vinegar and wash as usual. Do not ever put white vinegar into a washer with bleach, it creates toxic gases!