The 2013's ride way better than the previous-gen trucks.
I'm not certain how to improve the ride at stock height, but I agree with the tire pressure reco's above.
Air-up to tow, air-down when not towing.
If you're interested in a suspension system that provides some lift (either 3" or 6"), Carli Suspension has progressive coils and leafs, plus custom-valved King or Bilstein shocks. It's not cheap, but you get what you pay for.
My truck rode like garbage from the factory, it's a creampuff now.
It's still a heavy truck, but it's very compliant with 2.5-inch King shocks...and that's with stiff Toyo MT tires. Compared to stock, I'm 3" higher in front, about 1.5" in the rear, so it's just a little higher that what you get from a "leveling kit".
http://www.thecarlisuspension.com/
