As time goes on, couch cushions tend to wear out with repeated use. When a couch starts sagging, it's difficult to get in and out of it and often uncomfortable to sit on. A few do-it-yourself steps can be taken to preserve your sofa and stop it from sagging.
Wash the cushion covers
Remove the cushion covers from the sofa cushions. Read the cleaning instructions and follow them to clean the cushion covers. If you're able to wash the covers, the cleaning process should tighten up the fabric or shrink it just a bit so that when you return the cushions to them, the covers keep the foam taut. Move the cushions around so the same sections aren't always in the prime position for sitting.
Add foam to the cushions
Measure the sofa cushions. Cut sections of 1/2-inch thick foam to fit inside. Add the foam to the existing foam inside the cushions, making sure it is inserted evenly. If you need more support, add a second layer of foam.
Add plywood under the cushions
Remove the cushions from the sofa and measure the seating area. Cut a piece of plywood that is about 1 inch smaller than the seating area all the way around. Set it down on the couch, then set the cushions on top of it.