Soft foam cushions can make it difficult to rise from a chair. Heat and repeated compression can cause the cell walls in foam rubber to break down. These older foam cushions do not provide the same amount of support they did when they were new. Don't worry, you don't need to replace the seating, you can easily increase the stiffness of the foam rubber cushions on favourite chair or sofa.
Remove your cushion from its cover and place it in a plastic leaf bag. Fold under any loose edges and return the cushion to its cover. The plastic bag will slow the release of air from the cushion and provide just a bit more support. If you desire even more support you can tape the bag closed, but this can cause the cushion to rise unevenly.
Apply iron-on interfacing to the top and bottom of the cushion to add more support. Place the cushion (without a plastic bag slipcover) on the interfacing and trace around it with a pencil. Follow the manufacturer's instructions for applying the interfacing to the cushion.
Cut a piece of plywood to fit under the cushions to provide very firm support. Place the chair cushion, or all the sofa cushions, on the plywood. Trace around the cushion with a pencil and cut out the support with a circular saw. Sand the edges smooth with sandpaper to avoid snagging the upholstery fabric.
Trace the shape of your cushions on a large sheet of paper and measure their thickness. Have an upholstery or outdoor supply store cut new foam rubber inserts for your cushion covers. Many different densities of foam are available, so you can choose the amount of firmness you desire. This is the best way to restore old cushions.
Army surplus and fabric stores often carry foam rubber. Call ahead to see if they cut it to size.