What can I place under my sofa cushions to reduce sag?
3/4" plywood will do the job, but the outcome will depend on how the sofa's supporting structure is built. You'll want to cut the sheet to a suitable shape and bullnose the edges with a router or sander to prevent it from cutting through the fabric of the cushions and the base. It probably will eventually, though. You could also wrap it with old sheets or similar to protect the sofa.