Try lowering the pressure in your tires a little. Also, try to fit as large of a beanbag onto the seat as you're comfortable operating the tractor with (no, I don't mean one of those big ones you find in someone's living room!) - beanbags have a lot of cushioning effect - boaters use them when operating in choppy waters. Just make sure that you can easily reach all controls while on the beanbag, and realize that your CoG will be slightly higher.

Another technique is to slow down a little. That tends to remove a lot of jerkiness of these little tractors. You could also try smoothing out your lawn, but that is much easier said than done (can't just drag a barrel full of water over it!).

Otherwise, google "lawn tractor suspension seat" and you'll find some, prolly around $500 for anything decent.

I have lowered the tire pressure, but it still feels as if my spine has been hammered after a session. Our yard is rough, despite regular rollings with a water-filled roller.

I have looked at countless after-market suspension seats, but they all seem too high for this small lawn tractor.

Thanks for the bean bag suggestion. I was also tempted to try a wheelchair cushion.