is there a trick to wash a down comforter?

The trick is the drying, not the washing. Wash it in a big front loader (a laundromat one if yours is small - a top loader with an agitator can tear it), gentle cycle, a small amount of mild or down-specific detergent, extra rinse. Then dry on low with two or three dryer balls or clean tennis balls to break up the clumps. It takes hours. Pull it out every half hour and shake it. If it goes back on the bed even slightly damp in the middle, it'll smell and can go mouldy.