Check places like the 99 Cent Store for a product called Soil Love for the stubborn stains. With the MLPs I scrubbed them with an old tooth brush as well. A few of them needed multiple soaks with proper drying in between but most everyone came clean enough to pass on to other people with just Oxyclean and Zote soap.
There might be some secrets hidden in videos about old fashioned laundry routines. Wood ash, lye, ammonia and all that.