Well, this has to be just about my most favorite, and easiest "mod" I've done to any of my tools yet!
1) Cut the top 0.25" or so off the top of the mouthpiece
2) Take the little 3" drawstem from your MFLB (if you have one)
3) Insert into top of mouthpiece until it bottoms out and hits the metal flue of the cartridge the mouthpiece is on.
... voila! You get:
1) Visible vapor - I can see the drawstem while I'm taking my hit so I can gauge my draw speed by watching the vapor thickness
2) You can go for weeks without the mouthpiece staining/turning brown. I've been using the pic above daily for about a week now, and the outside is turning brown likely from my fingers touching and handling the mouthpiece after filling and changing carts, etc. The vapor does not come in contact with the silicon mouthpiece (well mostly), and instead travels straight up the drawstem about 95% of its path.
3) Hmm.. actually I guess just those 2 are it. But it's so easy, totally worthwhile imo!
http://i.imgur.com/8kZN9.jpg