Is it a Perspiration Stain or is it Deodorant.Stains..  

If you are cleaning a White item...  Always make sure it is Washable... If not your local dry cleaners will most likely be able to remove the stain for you..

Ok White Washable item..    Here is how i get the yellow out..

First dampen the area with water,  and make a paste with Water & Oxi Clean.   Spread the Paste on the yellowed area  and let it sit for 20 minutes...   Now start a large pot of water to boiling...   After letting it sit for 20 minutes..   place the item in the boiling water....  if you  do not want to submerge the whole item..  pour the boiling water over the the area.... if it is too hot for you to rub area..  us a old tooth brush.....  if this is a stubborn stain you may need to repeat this process...     .

The Boiling water helps to break down the oils in the stain..  

If this is a Colored Item..

Use 1 tablespoon White Vinegar and 1/2 Cup of water... soak for 20-30 and launder as usual.