To keep your duvet from shifting inside its cover while you sleep, secure it in place at the corners with twill ties.
Source: Martha Stewart Living Television
Start with eight 6-inch strips of narrow twill tape -- four pieces of one color and four of another. Hand-sew a length of one color to each of the top corners of the duvet, and sew a length of the other color tape to each of the bottom corners.
With your duvet cover turned inside out, sew a strip of twill tape onto each top corner of the cover, making sure the colors correspond with those of the sewn on the top corners of the comforter. Do the same on the bottom of the cover.
Tie each of the cover's corners to the matching corner on the duvet, then turn the cover right side out over the comforter.