Besides the fact that a couch with sagging cushions in unsightly, it is harder to stand up from one than from a couch that has a firm seat surface. You can rescue your couch from a pending trip to the local landfill by creating a new, firmer seat surface for your cushions. Add new cushion covers and a few throw pillows to get several more years of use before you release it to a college-bound child.
Remove your couch cushions. Carefully feel along the creases between the seat base, back and sides to see if there is a lip at each short end, which is created by the couch's existing framework. Most newer couches have one. Proceed to Step 3 if your couch does have them. Turn your couch upside down and remove any cloth covering the bottom to reveal the underside if it does not have a lip.
Measure the inside of the underside of your couch at the short ends. Cut two pieces of 1/2-inch by 1-inch stock wood to that length. Position the pieces against the inside of the short sides of the couch, so that they are 1/4 inch below the level of the seat base on the upper side of the couch. Nail them into place using 3/4-inch long carpet tacks, pointing toward the outside of the couch frame.
Cut a sheet of 1/4-inch thick plywood to the length plus 1/2 inch and the width plus 1/2 inch of the inside measurements of the underside of your couch. For example, if the inside measurements of the underside are 22 inches by 60 inches, cut your plywood to 22.5 by 60.5 inches. Use a table saw if you have one, or have the plywood cut at a lumber or home improvement store.
Brush rubber cement over the plywood sheet. Cut the memory foam pad to fit the plywood sheet and press it into place.
Cut muslin 6 inches longer and wider than the plywood. Fold the long sides down 1/2 inch and press flat with a steam iron on its highest setting.
Place the foam-covered plywood foam side down on the muslin with 2 inches of fabric overlap on all sides. Beginning in the center back along the long side, staple the fabric from the centre to the right corner every 2 inches, pulling the fabric toward the right corner as you go. Return to the centre again and work your way to the left corner.
Repeat the previous step along the centre front along the long side, stapling to the right first, returning to the centre, and stapling to the left. Repeat again for short ends of the plywood, working from the centre to the corner. Pull the corners tight, fold them under and staple them securely.
Turn the couch upright. Slide the plywood sheet into the creases between the edge of the back frame and the sides of the couch until it rests on the supports you created or on the lip created by the couch's existing back and side framework. The edge of the back frame supports the length of the board, which helps distribute weight more evenly to prevent sagging in the middle. The sides of the couch frame or the supports you created will provide the remaining support for the narrow ends of the board.
Replace your couch cushions on the now-firm seat bed, after making new cushion covers if you desire.