This is from the Queen of Cleans website.
Perspiration Stains
After wearing a garment for the first time, prior to laundering, work in some warm water and Fels Naptha® Laundry Bar Soap, available in the bar soap section of your local supermarket (usually on the bottom shelf, or in the laundry additives section), into the underarm area. This will prevent the yellow stains from forming.
If you have a garment that has only been worn once and odor is present, apply warm water to the area and work in 20 Mule Team® Borax, which is a very effective odor remover.
If you already have stains, try dampening the fabric with warm water and working in laundry detergent and Biz Activated Non-Chlorine Bleach. Allow this mixture to soak into your garment about 30 minutes prior to laundering. You can also try heated white vinegar on existing stains. Spray it on the fabric, then work in 20 Mule Team® Borax. This is very effective on any odor or stain that might be present. If the color has changed in the fabric, try spraying with sudsy ammonia, let it sit about 15 minutes, then launder as usual.