You cant just "tuck" the extra fabric in the seam between couch cushion and back, but we found sticking a long skinny piece of PVC pipe in there keeps the cover better in place.  It also would be better if it had a way to connect underneath the sofa.  We are finding that every time the kids sit on it, it slides up and has to be readjusted often.  The look and feel are great though so for a $40 couch cover its a good deal.