A duvet cover does a yeoman's job in protecting a comforter from spills, stains, and general wear and tear. To prevent the comforter from shifting around inside the cover, most need a slight modification. Sew two pieces of 2-inch-long fabric tape to all four inside corners of the duvet cover; at each corner of the comforter, push the filler toward the center, tie the fabric tape around the bunched fabric, and sleep happily ever after.
Fluffing a Duvet
Martha Stewart Living, February 1998