On Sun, 20 Jun 2010 14:15:31 -0400, mcp6453 wrote:

My elderly in-laws have a couch that allows you to sink deeply into it
when you sit.

Lift the sofa cushions. Some thin plywood (3/8) under the cushion
will give support if affixed properly.

An upholstery will cost you some dollars, even if you change the couch
springs.

Springs stretch. I sat once on a unit, the person, when I mentioned
broken springs, said .... but it "looks good"!