Just like my deer, I put on a low "boil" with oxyclean and a tablespoon of dawn dish soap and pressure wash every 4 hours over the course of 12 hours.
Every time I fill the water up I let it spill over the top so that the oils and grease leave the pot.
Costs about $10 in propane.
I dry all my heads and then paint them with white spray-paint so I don't worry about any yellowing.