Two things I did that firmed mine up alot:

Go to Menards/Lowes/Home Depot and buy carpet samples. The stiffer the backing the better. They are usually about 1.5' by 2.5'. I paid 4 bucks for two. Take your seat out, flip it over so you are looking at the springs, work the carpet samples under the springs between the springs and the foam. This keeps the springs from going into the foam.

If you are willing to take off your seat covers: I bought 1/2 or 3/4 inch thick high-density foam (green, if it matters) and used spray trim adhesive to glue this onto the top of the foam where you would be sitting. It was interesting trying to get the covers back on, they were very tight.

So there you have it, a couple of low buck ways to stiffen up your seats, these worked great for me....good luck.