Loft is warmth. Down insulates by trapping air in its clusters, so anything that flattens or clumps the fill — dirt, body oils, being stuffed away damp — costs you warmth. The goal of all down care is keeping that loft alive.
Wash rarely, but properly. A season or two of wear between washes is fine. When it's time, use a down-specific detergent (normal detergent strips the natural oils), run a gentle warm cycle, and rinse twice so no soap stays behind to mat the clusters.
The dryer does the real work. Tumble on low with a couple of clean tennis or dryer balls, and be patient — it can take a few cycles. The balls break up the wet clumps and the low heat fluffs the down back to full loft. Pull it out only when there are no damp lumps left inside the baffles.
Store it loose and dry. Never keep down compressed in its stuff sack long-term; hang it or lay it flat so the fill stays open. Air it out and give it a short tumble now and then over winter to keep it lofty.