I never have a sheet under the duvet - everything gets tangled up and shifts around. The duvet cover needs to be washed almost as often as the sheet so why bother! I have a king size + duvet and just lay the cover on the bed, stuff the thing into the top corners then the bottom ones, grab them and shake like mad until it shoogles about; then go to the sides and do the same. Very good exercise for the upper limbs as it takes at least five minutes to achieve
. As I am the only one in the 6' bed, I tend to move over to the other side after a few days so it lasts longer 