Wayne's away right now so I can sneak in this post...
As a former vendor of suspensions, I know somewhat of which I speak. Not to mention, riding in my trucks are like sitting on a cloud, suspended in whipped cream, on a bed of angel feathers, floating in an ocean of virgin's tears.
The difference is the right combination of springs and shocks. At first I thought it was just springs, but I've recently been converted to truley believing in the importance of excellent shocks to a great ride.
Shocks are very important. Be sure that the compression stroke is soft, and the rebound is harder. Adjustable shocks can really help in getting the right ride for your particular truck. Gas charged shocks MUST BE INSTALLED THE RIGHT WAY UP. Otherwise, the gas collects in the valve instead of the oil, and you got nothing, nadda, the shock doesn't work.
Ride problems are often mistakenly attributed to wrong number of leaves, or fiddling with the leaf pack. Indeed, this can change your ride height and spring rate, but the fact is that your shocks have a bigger effect on ride quality than spring rate, especially if the compression stroke is too firm. And don't overlook the effect of a siezed bushing or shackle on the ride. These often overlooked items can have significant effect on ride quality. And of course, there's always the 'stickyness' between the leaves on rigs that don't get articulated regularily. Ever wonder why your rig feels so much smoother at the end of the wheeling day than the beginnning?... the rust between the leaves has been knocked out.
The best and trickest shocks are monotubes, like those Pro-Comp MX-6. I have these on TippyR, and they completely changed the way the air bag suspension feels. On Ziplock I have those dreaded foam cell shocks from Belton. Most people hated them for the nasty hard ride they have, but in this application - a tallish sprung over truck on soft springs, they totally kick ass. They have the heavy damping needed to offset the soft springs and the natural roll induced by the height of the vehicle.
For a street rig, I would always recommend a gas charged shock, like the monroe gas magnum, coupled with the right set of leaves.
That's all I have to say.