Increase the firmness of sofa and chair cushions without replacing the foam interior. Foam rubber deteriorates with time and wear. The cells in foam rubber break down over time and allow air to escape from the foam more easily. Repeated compression bends the foam out of shape. Encase the foam in plastic to hold the air in the cushion and improve the firmness. You can also add extra support under the cushion.
Remove the foam rubber from the cushion cover. Place the foam in a large plastic bag.
Remove as much of the air as possible from the bag be pressing it against the foam on all sides.
Duct tape the bag closed.
Replace the cushion cover.
Remove the cushions from the furniture.
Trace the shape of the base below the cushions on a large sheet of paper. You may leave off the arms of the "T" on T-shaped cushions.
Use the tracing as a pattern to cut a support from a sheet of 1/4-inch plywood.
Sand the edges of the plywood smooth to prevent snagging the fabric on the furniture.
Place the plywood under the cushions.
Use a vacuum cleaner with a hose to remove the air from the plastic bag.