The best method that I have heard of is to fill your pipe with uncooked rice and water and then shake that shit around and dump it out in the toilet or wherever. After you have done this a few times, soak it overnight in nailpolish remover because nailpolish remover is nonpolar and THC is polar. I've never actually soaked my piece overnight, but after about 5 minutes of soaking it in nailpolish remover, the liquid turned from a clear to a nasty yellow. After you've let it sit for a while, rinse it out with warm water and uncooked rice again, or you could try to boil it, though I've heard that boiling glass weakens it.

Good luck, and let us know how it turns out!