Drop your couch cover into your clothes dryer with a damp rag. Set the machine to cool or warm--not hot--air and run it for 10 minutes to remove wrinkles, which in turn reduces the tendency of the fabric to bunch.
Attach a piece of rug grip just smaller than the dimension of the cushions to the bottom of each with hook-and-loop tape to reduce sliding action when people sit down and stand up.
Tuck the ends of the couch cover up under the arms of the couch, as well as firmly under the cushions and the back of the couch. Tuck as far down and back as you can without exposing any of the couch upholstery--the tighter the tuck, the more secure the cover.
Slide a dowel rod in the crack where the slipcover is tucked into back of the row of seat cushions, pushing it down so it is hidden from view to weigh down the tucked material and prevent slippage.