I watched my auto trimmer at work a few days ago and noticed that once he had fitted a section of leather to the bolster of a car seat, he used a heat gun to gently heat the leather from the center of the bolster outwards whilst stroking over it with his other hand. I asked him what he was doing this for and he tells me that when fitting the leather, it is never 100% smooth and tight over the inner foam, so by heating it gently and smoothing it slightly shrinks the leather and ends up being a smooth firm fit. He did say to not heat directly onto the stitching though as the thread used is nylon, and it will either melt or shrink more than the leather causing the thread to break or "wrinkle" up at the seam.
So for any of you that have seats (car seats...
) which show signs of "floppiness" due to stretch...get the hair dryer out and gradually / gently warm the leather so that it shrinks a little and tightens up again... 
