Don't sweat it. Those yellow stains in the armpits and around the collar of your favorite white T-shirts aren't a sign that you're sweating too much or not cleaning properly.
These areas are just harder to get clean and are made up of more than just old perspiration and dirt.
Undissolved deodorants may be a culprit, too. If you have hard water, the deodorant residue (and soap and perspiration) can't wash out properly. Here is a method for removing those sweat stains.
Mix 1 quart water with 4 tablespoons salt. Sponge this mixture onto the stained area, then repeat until the stain disappears. Wash as usual.
A Guide to Stain Removal
Stains need all the help they can get. Salt is there to lend a hand.
Blood: Soak a blood stain on cotton, linen, or other natural fiber in cold saltwater for 1 hour. Wash using warm water and laundry soap, then boil the fabric in a large kettle of boiling water. Wash again.
A fresh blood stain should disappear easily if it is immediately covered with salt and blotted with cold water. Keep adding fresh water and blotting until the stain is gone.
Gravy: Try covering a fresh gravy stain with salt and letting it absorb as much of the grease as possible. A stubborn stain may need a 50/50 solution of ammonia and vinegar dabbed on and blotted until the stain disappears.
Grease: Remove a fresh grease spot on the fabric by covering it with salt. Wait for the salt to absorb the grease, then gently brush the salt away. Repeat until the spot is gone, then launder as usual.
Double-knit fabrics can be a stain challenge when it comes to grease. Add 1/2 teaspoon salt to a small dish of ammonia, and dab the mixture directly onto the grease spot. Let sit, then wash as usual.
Ink: Rub salt onto a fresh ink stain on fabric, and soak the fabric overnight in milk. Wash the fabric as usual.
Mildew: Make a thin paste of lemon juice and salt, then spread the paste on mildew stains. Lay the clothing item out in the sun to bleach it, then rinse and dry.
A mixture of salt, vinegar, and water should remove mildew stains on most fabrics. Use up to full-strength vinegar if mildew is extensive.
Prevent mildew growth on shower curtains by soaking them in a bathtub full of saltwater (½ cup salt into the tub). Soak the curtains for several hours, then hang them to dry.
Wine: Remove a wine stain from cotton fabrics by immediately sprinkling stained area with enough salt to soak up liquid. Then soak the fabric for 1 hour in cold water, and launder as usual.
As you've seen, salt can be a vital asset in the list of laundry ingredients. See what salt can do for you.