I cork the neck up to the ferrule's edge as normal. I then separately cover the ferrule with a thin strip of material so that it is a snug (sliding smoothly but with a decent seal) fit inside the mouthpiece shank. Usually a strip of the thicker sheet teflon (attached with super glue naturally...) and gently sanded down as required with an 800 grit sandpaper strip works fine. On one ferrule I had to use a thin strip of tech cork since the gap was too large for the available stock of sheet teflon I had on hand.
Good point- it takes just a couple of minutes to do, is unlikely to harm anything, and and to the extent that a small gap at the interface between the neck end and the mouthpiece shank causes a problem it addresses that problem. There's also Martin Mod's "zinger ring" theoretic effect.
http://cgi.ebay.com/MartinMods-Mart...492?pt=LH_DefaultDomain_0&hash=item3a50eabdec
I figure the sheet teflon is about as reflective as brass would be.
I've not heard an audible difference in use after the initial "this is better than sliced bread" novelty aspect wore off with or without the strip but it's a small tinker and so I do it anyway.
Bad point; the covered ferrule is pretty accurately sized to the mouthpiece shank. Using a more tightly shanked mouthpiece could wind up with a binding effect- though grease and minor twisting muscle generally render this a non issue in practice.
On regular non ferruled necks I paint the face of the cork on the end with nail polish to enhance reflectivity but again, figure it's more voodoo than acoustic science.