Storing it loose is good advice, as is drying it after use.

Down is remarkably resilient, though. I have a Moonstone bag that was stored stuffed for well over a year and within an hour of unstuffing was back to full loft (I have too many bags and not enough space.) This leads me to believe that it's probably OK to store a bag partially compressed, that is, in a bag maybe three or four times the size of the stuff sack. I keep one of those "Moisture Grabbers" (desiccant bags) in with them, too.
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Always remember that you are absolutely unique, just like everybody else. -Margaret Mead