Tried the search feature but I couldn't find anything about this so I thought I'd share this. This is used mostly by healers in Arenas, I use it like crazy anyhow. What you do is wait until you're almost out of combat (Takes 5 seconds for combat to fade) and then you start casting a massive heal let's say Holy Light or Greater Heal. While it's casting spam your drink button and due to lag, when the cast is finished you'll sit down and drink assuming you timed it right. Obviously doesn't work when you have a pet on you.
Good luck ;D