Whether your couch is brand new or it has some wear on it, the cushions on it may be too soft. Couch cushions typically contain down-fill, high-density foam or polyester fill. These materials are meant to be soft for comfortable seating. Sometimes, though, they are too soft, and you sink down into them when you sit down. It may be comfortable, but it's hard to pull yourself up when you attempt to stand back up.
Remove the cushions from the couch. Measure the width of the couch (the springs) from the left side to the right side. Now measure the depth of the couch from the back of the springs to the front. Mark down the measurements.
Mark out the measurements of the couch springs on a piece of thin plywood--1/4-inch plywood works well for this purpose. Use a power saw to cut out the piece you need to firm up couch cushions.
Stretch out a piece of medium-to-heavy material on the floor. Any material that will cover the rough edges of the plywood so they don't mar your couch will work fine. Examples include fleece, microsuede and weaves. An old blanket will also do the trick.
Measure and mark the material (with seamstress chalk) so it's large enough to wrap around both sides of the plywood. Cut the material to size with a pair of scissors.
Place the piece of plywood onto the material so the widest side is on the widest end of the material. Hold the material onto the plywood as you fold it over and wrap it.
Place the thin, wrapped plywood onto the couch springs. The material will protect the couch and the cushions from the rough plywood edges. Replace the couch cushions.