Hit the bumps slower:B
As mentioned, you could probably do some suspension work (shocks, air bags, helper springs), though you would probably change the feel of the ride when you are not going over bumps (most of the time, I assume), so not sure if you really want to make the changes.
What is the air pressure in your tires? Is it set at the recommended pressure for the weight on them? Or did you just "max" them out at the recommendation on the side of the tire? Over-inflated tires will give a rough ride too.
Good Luck:)