foam pool noodles inserted in seat pad. Amen!
 

I took a $3 foam pool noodle and cut it in 3 pieces long enough to stretch across my driver's seat bottom (under the MB Tex of course)

My seat cover comes up easlily, so without removing the seat from the rail, I just lifted up the seat covering and inserted the foam sections across the frame and between the seat springs in the front.

I can't believe how good it worked. My wife even said she enjoyed driving the car this week due to the new seat feel, and the throttle coupler I replaced so there is no play in the "GO' pedal.

I would try it if you have a sagging seat bottom that has no firmness left....