Originally Posted by VMan
Here's what I do, it works like a charm. It has been effective on many types of stains, including sweat stains that have been present for several years.
1. Soak the shirt for an hour or two in a solution made from one gallon of hot water (as hot as it will come out of the faucet) and one cup of vinegar. This just helps to loosen the stain before washing.
2. Purchase a small tub of Oxy Clean (get the granules, not the liquid) from the store. Mix a strong solution of this using four to six ounces of hot water and one scoop of Oxy Clean. The solution should be like a watery paste.
3. Rinse the vinegar from the shirt. Generously apply the Oxy Clean solution from step two to the yellowed areas of the shirt. Place the shirt in a plastic pail and let sit overnight.
4. Remove shirt from bucket. Mix a solution of 1 gallon hot water and one and a half scoops of Oxy Clean. Allow shirt to soak in this solution for up to 12 hours.
5. Remove shirt from solution and briefly rinse. Wash shirt using regular detergent, and rinse. Voila.
I've done this on many shirts with almost complete success. A few shirts with really bad stains may take two tries - usually the first treatment removes about 90% of the stain, leaving a really faint yellow mark behind. The second time usually removes this.
Deliberate necropost: I followed Vman's procedure here, and it works wonders. I had some undershirts that had really, really nasty caked-up deodorant stains, and it got them out beautifully.
Highly recommended to anyone with shirt stains--especially the underarm ones from aluminum-based antiperspirant, which are nearly impossible to get out with normal laundering.