Most down insulated garments can be machine washed in cold water, and then tumble dried on low. Add a couple of tennis balls to fluff up the garment. Be sure to check the label on your garment before you wash or dry it, as it may have different instructions. You can also check out Patagonia's care guide for down insulation. See the Patagonia Product Care Guide.
Best way to wash down garments is to wash normally (cold water, medium or delicate cycle). Then the trick here is to AIR DRY!!!! Lay that jacket out for 24 hours, THEN fluff it in the dryer, you won't even need heat.
When you let the jacket air dry, the down clumps much less than if you dry it immediately. Less fluffing in the dryer means less wear and tear on your jacket and a longer, happier relationship.