Wash the comforter with the machine set on the gentle or delicate setting with two rinse cycles, using cold water. Add a small amount of mild or all-natural detergent. Too much soap can strip the down, so be sparing. Dry the comforter in an extra-large capacity dryer on the lowest temperature setting possible. This will take 2-3 hours, but you want to make certain that the down is completely dry, to prevent mold growth.
Wash your down comforters once a year at most. To keep them clean in between washings, cover them with duvet covers made of tightly woven material, and wash the duvet covers as often as needed.