I bought a used 2005 Silverado 2500HD with 74K miles. I knew the seat was a bit sagging when I first bought it (and used that to my advantage in negotiating). Now I'm trying to figure out what to do with it. When I drive over bumps, the seat launches me into the air. While fun, it's not the most comfortable or the safest. Any ideas on how much I can do to firm up the seat? I'll probably take it to an upholstry shop to check out, but wanted to get thoughts first. Should the springs be replaced, just add padding, ....?

The seat is full manual adjust and the truck is extended cab, so the seat belt is integrated into the seat.