If You Don't Have A Health Care Directive
Fill one out. There are a bunch of them on the Internet, free. Here's the one I used.
I filled mine out, had my neighbors witness it, and had them initial this little doohickey I added at the bottom:
I do not want to live as a human turnip, and I’ve had a great life and prefer that I be let die instead of forced to live on as a body in a bed being kept alive by machines if that becomes the case. Don’t be sentimental! Let me go.
And then I listed three possible friends with medical/epidemiological/research backgrounds to contact for medical advice.
I made five copies of this, filed the original, and shrunk down a little version and put it in my wallet.
You can also register your living will here for $125. An explanation of how that works is here.