Shake your duvet out every morning as you make the bed. This keeps the down even and fluffy.
Hang the duvet on a line outdoors to freshen it up. You can do this once per month or as needed.
Protect your duvet with a duvet cover. It is much easier to wash and clean a cover than the actual duvet.
Wash your comforter with a special-purpose down comforter soap. Down duvets should be washed in cold water on the gentlest cycle available, then dried on low heat or hung out to dry.