You know what's better than knowing how to get rid of stubborn underarm stains? Keeping those horrid yellow stains away in the first place. Real Simple tells us we can use baby powder as a stain guard for our clothing.
Sprinkle a little on the shirt's underarms and collar, then iron to prevent sweat stains on white shirts. The powder forms a barrier that keeps oil and grime from seeping into the threads.
Unfortunately, it looks like you'll have to repeat this after washing the shirt, but if you're going to iron it anyway, keep some baby powder on hand. As they say, an ounce of prevention is worth a pound of cure.
Baby Powder as Stain Guard [Real Simple]