The best way to clean a down comforter is to do it as infrequently as possible. The natural oils in down break down when it is washed, as does the loft of the down. Excessive cleaning will shorten the life of a down comforter, as well as making it lumpy and flat. Most manufacturers recommend that consumers clean their down comforters every three to five years. This is why it is extremely important to invest in a comforter cover, and to engage in routine care which will keep a comforter smelling fresh. Obviously, you should clean a down comforter if it has been subject to an accident or spill, ideally as quickly as possible so that mold does not set in.
Ideally, you should have two layers of coverage for a down comforter. The first is a tightly woven zipper cover in a neutral color. The second is a decorative cover. Both of these covers can be washed as frequently as you like, and you should follow the laundering directions on their tags. Down bedding should also be fluffed on a daily base, and you should avoid sitting or lying on down, since that will crush the delicate feathers.
Airing is also an important part of down care. In warm weather, try to take down bedding out of its covers and outdoors at least once a week. Hang the bedding so that it is exposed to plenty of breezes, ensuring that it is completely dry and fresh smelling. In addition, exposure to sunlight will eradicate any mold or mildew spores which may have moved in. In less favorable weather, air down whenever you have an opportunity to do so.
When it is time to clean a down comforter, take the time to read the directions from the manufacturer. If the manufacturer specifies that the comforter needs to be dry cleaned, take it to a professional dry cleaner who is experienced with down. Do not clean a down comforter which should be dry cleaned with water, as the water may destroy the down. Ask around for dry cleaning recommendations, since not all dry cleaners can handle down.
Even if your down comforter is machine washable, consider using a professional cleaning service. If you dislike this idea, find a laundromat or a friend with an industrial sized front loading machine. Agitator machines can damage down, as can small machines, so do not use a regular home washing machine. Wash the down comforter in warm water with mild detergent, and do not use colored detergents or bleach. When you take the comforter out, gently press it to remove water, and dry it on a low heat setting with several tennis balls to maintain the loft of the down. Check the comforter frequently as it dries, since you do not want the comforter or down to be burned, and plan on a long dry time. When you clean a down comforter, it is vitally important that it be completely dry before being put into storage or back on the bed, and this can take up to two hours.