A friend of mine put plastic liners between his leaf springs and it made an incredible improvement in ride quality. Other then that an anti sway bar and radials on a well maintained truck are your best low dollar, mostly stock, mechanical improvements. As far as non mechanical improvements go I found that an early to mid '90 full size Chevy bench seat made a world of difference. It's more comfortable, doesn't bounce you around the cab like a pea in a tin can, and absorbs a LOT of road noise.
