Step 1
Refill the couch cushions. Add extra foam to the cushions, or completely remove the old foam from the cushions and replace it with new, sturdier foam. A sheet of foam can also be cut to fit and placed underneath the cushions to provide some extra support.
Step 2
Call a professional upholsterer. The springs could be old and sagging, and an upholsterer can remove the old springs and put new ones in. This would be less expensive than replacing the couch and should add some firmness to the cushions.
Step 3
Stick a piece of plywood underneath the cushions. The plywood can provide support without having to reupholster the couch. A similar solution would be to use an egg crate foam cushion underneath the cushions. Both of these items are firm enough to harden up the couch and make it more supportive.