re- terri schiaus, page-12

There's a group of forms callectively called "The Advance Health Directive" which is obtained (or borrowed to be photocopied) from one's GP. I think there's also a gov. URL which has it.
This group of forms asks you what you would like your doctors and hospitals to do in certain emergency situations. It gives you the opportunity to allocate an attorney (not necessarily a lawyer) who will oversee all your final treatment.
You fill those forms up in the presence of your doctor, get your witness and attorney to co-sign them, give a copy to your doctor and one to your attorney and as many as you like to others.
Bingo! It's guaranteed that you won't be kept in a coma for 15 years.
With this bunch of forms, one can allocate organs and the whole corpse to which ever institution wants them and, if he or she changes his/her mind later, no probs, just fill another bunch of forms adding whatever caveats you like.

It's one reason why no doctor would intervene in Schiavo's case and certainly Bush had no right to do so.