Although a duvet cover does an admirable job of protecting a comforter from stains, it is notorious for being a shifty character. Keep it and your comforter neatly in place by turning the cover inside out and sewing two pieces of five-inch-long fabric tape to all four corners. Then tie the fabric tape around each corner of the comforter, and sleep tight.
How to Keep a Duvet in Place
Martha Stewart Living, January 2011