Step 1
Insert a piece of plywood under the cushions. This inexpensive fix works well if you're on a budget. Cut ¼-inch plywood to fit underneath the cushions, or have a home improvement center cut it to size for you. Sand the edges to prevent splinters, or cover the plywood with fabric. You can purchase similar commercial products at big-box retailers and online. Usually constructed of MDF (medium-density fiberboard) and covered with vinyl or fabric, they come hinged for easy folding and storage.
Step 2
Add a layer of high-resiliency foam underneath the seat cushions. Foam rubber comes in different grades of firmness, with "high-resiliency" being the firmest. Find the firmest one you can at a fabric or upholstery retailer and lay it underneath the seat cushions. A thickness of 1 inch usually suffices. This may not be the best fix in terms of aesthetics, as the layer of foam might peek out a bit as it lifts up the cushions, but it does supply additional support.
Step 3
Remove the cushion covers and replace the internal foam or polyester fill with new, firmer material. For even stronger cushions, wrap thick foam pieces with sheets of Dacron. Hair fillings provide the firmest support of all. Upholsterers in previous centuries often stuffed cushions with horsehair; if you've ever sat on a Victorian sofa, you will appreciate the unyielding nature of this material. Modern upholsterers prefer a mixture of hog and cattle hair and other fibers, which provides firm, but more comfortable, support. You can find hair filling at upholstery supply shops. To construct the cushion fillers, sew muslin fabric into a box the shape of your cushion, leaving a few inches open on one side. Insert the hair filling to your desired firmness and sew the opening closed. Replace the cushion cover.
Step 4
Take your sofa to a professional upholsterer who can reinforce or replace the springs. The best quality springs are referred to as "eight-way hand-tied," meaning that workers secure them to the sofa frame and to each other at the front, back, sides and diagonally with strong twine. If the springs lose this support, the sofa sags. This is the most expensive option, because usually it means reupholstering the entire sofa, but it results in the longest lasting solution.
Step 5
Replace the webbing in sofas that have this type of support. Webbing consists of strips of nylon, jute or polyester, usually about 3½ inches wide. Upholsterers tightly weave them back and forth and attach them securely to the sofa frame to support the springs. If the springs are in good shape, replacing just the webbing may cure the sagging problem. Call a professional for this job, unless you have upholstery experience, because you must tighten and secure the webbing to the frame properly in order to provide support.