Avoid Top Loading Washing Machines for Down Jackets and Down Sleeping Bags Too
This techinque should be a reliable way to clean stains from down comforters without damaging them. Many people make the mistake of putting a down comforter into a top loading washer with an agitator, which can tear the down comforter or break down the interior separator pockets that keep the various sections of down separated withing the comforter.
I remember actually having to go to a local laundromat to do this because I didn't have access to a front loading washing machine.
Air Dry Down Comforters and Down Jackets
The other common mistake that people make is not air drying. Putting a down comforter or sleeping bag directly into a dryer can severly damage the items by degrading the down. In fact, the down will lose some of it's insulating and warming ability if it is damaged in a dryer. This is very important to avoid if you are washing a high quality item that is used in the back country for survival like a down sleeping bag or jacket for climbing and skiing expeditions.