Best thing I have found with the least amount of hands on is white brite powder. I fill a bucket with water and add the white brite powder and let it sit overnight. Then throw it in the washer. White brite is about 4 or 5 bucks.
I second the Oxi-Clean overnight soak... been doing this for a few years now. Works great
We put a scoop into a 5-gal bucket, fill up with hot water, drop in the pants, soak overnight, then wash as usual in the machine the next day