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Maybe try the Seat Saver? Fairly cheap, and probably more comfortable than plywood.

If you don't want to get into the innards of the springs and webbing, you can get a thicker and firmer piece of foam from a local upholstery shop. It's not cheap but it may work if the entire foundation isn't sagging. I did this for an elderly couple who had knee problems and they couldn't get up and out of their soft cushioned sofa. The cushions are REALLY firm but it is a non-invasive fix. Be sure to insist that your upholsterer gets you the firmest foam.

Sure you can get firmer cushions, but they'll just continue to sink on the worn out springs - and you can get a board to go under the cushions, but that will just cause the cushions themselves to wear out faster.

For a permanent and durable fix, there's not alot you can do without having the sofa redone.

A good upholsterer can repair/replace the springs without having to reupholster the sofa. They can go in from the underside to repair or if they have to remove the existing fabric they could reapply it.

Hey, I used the plywood trick for a good 10 years in my college and post-college poor years. Depending on the dimensions of your seat cushions, you can buy the foam innards at Joann Fabrics. Wrap it in a cotton batting to soften the edges. For that price, who cares if you have to replace it every 5 years?

Also, that's a good trick if you have a misbehaving cat who pees on the couch.

Be sure to get 2.5 or higher density foam
2.5 pound foam costs about 50 bucks for a 24 x 24 x 7 piece here in LA.

Might cost more because of the economy.

*This* is why not to buy used upholstery! I know, I know: re-use, recycle, blah, blah, blah. I'll do that with things I can sterilize, and things that won't go to bits on me because I can't tell how they've been used in the past.

Why not try to shore it up inexpensively and then if you can't stand it, you're not out that much money. Then you can decide if you love it enough to warrrant the expense of reupholstering it. Or.....you could reupholster it yourself.

In good upholstery the springs and the foam/down work together to provide stability and softness. When the springs are shot, they are shot. You are never going to feel stable on the sofa. You can fake it for a while by using some high density foam but since the foam does not have the springs to support it, it will break down fairly quickly.

~Lorrie @ MyDesignSecrets.com

I had a purple velvet sofa that I couldn't bear parting with after it started sagging in the middle. I did the plywood number and it really did do the trick. Later on I put new high-quality foam in the cushions and that ran be about $300, supplied by a upholstery professional. I think the sofa was around 6 feet long, if that helps. I strongly advise that you have the upholstery people put the covers back on the foam. It was a PITA to get on straight by myself. Good luck!

I actually did this with a sofa recently, ie. put a ply in to eliminate the springs. and its great if you like a firm ride.
Just make sure that you do a layer of high density foam on the ply base and then a medium density layer (2") above that to soften it a touch (2"). It may require a fabric change too though, depending on the method used to put it in place originally.

Try a couple of layers of dense egg crate foam if you don't quite want the rigidity of plywood.

You can also try using a higher density cardboard instead of the plywood. I had an old futon frame where the bars were killing me. I dropped in 3-4 layers of gatorboard (like your cereal box - but 1/4" thick) It was firm, but not hard, and soft, but not saggy.