Hometalk :: How To Fix a Saggy Sofa A professional upholsterer also offered this tip, in the comments: Cathy KochendorferEagle River, AK As a professional upholster, you can put foam in large plastic bag, use a shopvac or home vacuum and suck air out, flattening the foam. Slip on the cover and then turn off vacuum. Foam will expand to original size, remove plastic and zip up closure. Hope this helps!
This is genius! Fix your lumpy, saggy couch instead of buying new. So much cheaper: about $36! No sewing! Why didn't I think of this before. No more flat cushions! How to Revive Your Saggy Couch - An Exercise In Frugality