The essential thing is to have a Health Care Proxy who knows quite thoroughly what you would find necessary or unnecessary, tolerable or intolerable. The "piece of paper" is indispensible because it provides evidence. But even the "piece of paper" isn't going to save you, if you don't have a living person there to to actively and intelligently advocate for you.
That's why I recommend the NRLC "Will to Live" document.
Absolutely correct. The best solution is to assign a medical power of attorney to someone you know and trust (Mrs. Tick in my case) who knows your desires. Far better than any piece of paper that is left to "health care professionals" to "interpret".
COROLLARY: be careful to stay on loving good terms with your medical attorney-in-fact! :-)
I frequently remind Mrs. Tick that it is not acceptable to *remove* my life support before I am actually *on* life support.