A quality down duvet (sometimes called a comforter) will last a long time if properly cared for. With use, down tends to clump inside the duvet, making it lumpy and lopsided. Daily fluffing helps your duvet stay feeling new.
Dry your duvet on low/no heat/fluff cycle in your dryer (the setting name depends on your dryer).
Remove the duvet and carefully shake it to redistribute the down every half hour of the dryer cycle. You can also add two clean tennis balls or dryer balls to the cycle instead of removing the duvet to fluff.
Fluff the duvet daily or as needed. Carefully shake the duvet until the feathers (down) are evenly distributed and no clumps remain.
Air dry down duvets outside on a clothesline every six months to kill dust mites and freshen the down. Air drying in cold weather kills the mites. Fluff the duvet as directed above. If you cannot fluff the duvet and feathers remain clumped, it may mean the down in the duvet is wet and will need to be dried thoroughly.
Do not dry clean your down duvet. The chemicals used in the dry cleaning process can damage the fibres and mat the down together.