Despite what Cameron Diaz says, underarm antiperspirant/deodorant is a useful entity. But if you are a regular wearer, you know the resulting stainage and buildup on your clothing can ruin a good garment. Why the heck are those stains so stubborn? They're biological - it's science. It's all about the chemical reaction between the sweat and the stick. Even if you use the gel or spray antiperspirant/deodorants, you're clothing will fall victim. Like the sun rising in the east, setting in the west - it's gonna happen.
You can rub and scrub and even bleach those stains, but you'll succeed only in disintegrating your clothing. So how do you remove the stain? You've got to reverse the chemical reaction. It's easier than you think. Today's tip - de-gunking your underarms.
There are a couple of ways to attack those underarm stains, keeping in mind that if you've allowed it to degrade to the point of that hazy yellow/green appearance, it may be too far gone.
Operation Vinegar: Soak the stain in vinegar. Let sit. Scrub gently with a toothbrush or other laundry stain fighting brush. Let sit again for a couple of hours or even overnight, then work in your laundry detergent in similar fashion and wash according to your garment directions. If the stain is particularly cakey, make a paste with baking soda and vinegar and work that into the stain.
Operation Ammonia: Substitute ammonia for the vinegar in the above equation, but cut it with equal parts water. Keep in mind that it's a good idea to use ammonia in a well ventilated area, and NEVER mix ammonia with other cleaners.
Operation Lemon: Squeeze lemon juice directly on the stain and massage in a little table salt. Let sit and then launder.
Operation Discard: Throw away the shirt. That'll get rid of it, for sure...permanently.
I have heard that aspirin is great for this, as well, but it's never come to that for me, so I've never tried it. I guess you just crush some aspirin and work it in after making a paste with hot water.