I think once they start to go, you are fighting a losing battle. You can prolong it by regluing. The B-I-L uses a spray in/on product that I think is 3M. We used it on a neighbors car that was sagging badly. Sprayed it up onto the ceiling through the opened sides, then patted the liner up there, then went over it with soft plastic bondo spreader.

Big Truck: 00, 3.5, Endeavor, 5-Spd drive line in hand!
Little Truck: 87, 2.6T I/C, MT, LSDs, Tonneau Top
Her Truck: 03, 3.8, 20th Anniv, 65k
Daughter's: 06 Eclipse, Keeping it Mitsu!
FSMs: MitsubishiLinks.com