I've used this method for stubborn stains quite successfully, taken from PutThisOn (they took it back from StyleForum).
Soak shirt in a solution made from one gallon hot water (as hot as it will come out of the faucet) and one cup of vinegar. Let the shirt soak for 30 mins to 2 hours.
Rinse shirts, and squeeze out excess water. Empty bucket and rinse. In a cup, prepare a concentrated Oxy-Clean solution. Make...