I've posted this before but, here goes again.
Keep 3 or 4 spring clothes pegs in your bedroom. When putting on a clean duvet cover, push the top end of the duvet into the top seam of the cover, and into the corners, and secure with the clothes pegs (one at each corner, more along the top edge, as you wish). You are now free to pull the cover down to the bottom of the duvet, knowing that the top will stay in place.
