Does anyone have any clever ideas for how to keep a duvet neatly inside its cover?!?

I know I can't be the only one who gets really frustrated with this! Duvets which move around inside their covers, resulting in (usually) ending up with just the thin cover at the top and the duvet moving down inside.... I've tried sewing a button on each corner of the duvet and a loop on each inside corner of the duvet cover, but that didn't work. Press studs don't hold it but come undone. Velcro patches don't hold it either.