Goodbye to Yellow Armpit Stains: one part Dawn dishwashing liquid + two parts hydrogen peroxide + some baking soda. Use a small laundry brush to work the mixture into the stain for a few minutes. Let sit for an hour or so then launder as usual.
MS: DIY Underarm Stain Remover: 1 part Dawn (or any dishwashing liquid, I assume), 2 parts hydrogen peroxide, then add baking soda to create a paste. Use brush to work it in, and let it sit for 1 hour.