Which is in my top 5 least favorite household chores, by the way. Take some ribbon, cut into eight pieces about 1” long, and sew one piece each to each corner of your duvet, and one each to the inside corner of your duvet cover. When it’s time to put it on, turn the cover inside out, tie the top two corners together, flip it right side out, and reach in and tie the bottom ones. No more migrating duvet, and no wrestling the damn thing back into the cover.
I think that may have originally come from Martha Stewart.