First off, this does not work on ALL spells. It works on alot of them, and heres a list of the few that I know it does work on:
Lay on Hands, Innervate, Rebirth, Lightwell, Blessing of Protection, Divine Shield.
Its really simple to do also. You need either a player or a mob with an interrupt (Counterspell, Kick, Feral Charge, etc.). A normal silence will not work.
What you do is have the spell you want to reset on cooldown. Get a player or mob to interrupt any spell that you cast. Afterwards, just zone in or out of an instance, or log out, and the spell will no longer be on cooldown.
Easiest way to test this is to go to SM Cath and keep casting something until the first few mobs in there interrupts your spell, and then you can just zone out and your cooldown will be back up.
I searched and do not believe this is a repost, but if it is, my apologies.
EDIT: Video: cooldownz.wmv - FileFront.com