If you have access to a stairway with a
bannister running around the top, I highly recommend you
take advantage of this.
Basically you can stand, having stuffed the corners into the far end, hold it over edge and shake it so the fleecey duvet bit goes into the cover.
Now that it's mostly inside, hang the duvet along the bannister sideways and tug randomly at it straightening the duvet inside the cover.
Again, this isn't too different from the methods mentioned above, but the staircase allows you to shake it out to its full length.