Inspired by
@ataxian, for those like to add smaller restrictive mouthpieces to the mothership-style pieces (and probably others), check out this 50¢ deal:
http://www.vaporstore.com/proddetail.php?prod=mouthpiece-2nds. For being so generous we should buy out their inventory.
I use a bit of silicone tubing for the seal, layered however thick necessary. Great for social situations and so cheap everyone can have their own. Also worth saying, the smaller opening and resulting mouth embouchure presents more flavor on the inhale if that's what you're after (I switch back and forth).
[If I was more creative with pictures, here's where I'd post a shot from my morning sesh with some great books. In addition to Hitch's reading list, I'll suggest another from Sam Harris on his website.]