The correct technique is to replace a duvet cover by going to the store and purchasing a comforter. Ideally, purchase two comforters and alternate the one you’re washing with the one you’re using so that the bed is never without one.
I think there's a gap between the "bedroom is center of home" people and the "bedroom is for sleeping" people. For the former, they value things like changing the appearance of the bed for seasonal or taste-based reasons. For people like me in the latter category, the things on the bed are like the floor mats in my car: as long as they're clean I don't give a damn what they look like. They're purely functional objects that I don't even see for more than a few seconds a day.
:-)