If anyone has ever had a down comforter covered by a duvet and had it bunch up on one side or down at the bottom of the cover, this quick fix tip is for you. All you need is a handful of safety pins.
Pull your duvet cover inside out and lay it on the ground. Match up the comforter by laying it on top of the duvet. Then, start safety pinning every 12-15 inches or so around your comforter.
Finally, pull it inside out and button/zip the end up as normal. You won't even notice the pins, except how great it is to have your comforter stay in place.