How often do you wash your duvet? We don't very often in our household. According to Livingetc, however, it should be done once a year in a washing machine. Maybe that's the project we'll undertake during this blizzard.
Livingetc recommends washing on cold, with a small amount of detergent, and then tumble dry on low with some tennis balls.
eHow has more extended instructions, including a pre-soak in vinegar and water, and stopping to dryer to fluff every now and then.
We would add that you should check the label first, and make sure it doesn't need to be dry-cleaned. Livingetc notes that a double duvet might be easier to clean at a laundromat.