So here are my slipcover tricks:
It's as simple as rubber gloves and clothespins.
After washing with soap and a bit of Oxi Clean in a front load washer.
The rubber gloves seem to speed up the handling and maneuvering of the fabric over the furniture. The provide a better grip then bare hands can manage.
The clothespins hold the corners in perfectly and I keep them in place overnight so that the next day I have crisp corners.