Truthfully, putting it in your living will is your best bet. (You have one of those right?)
But... give your health care proxy some discretion.
Sarah wanted to be an organ donor. I wanted to honor her wishes. But it would have meant reversing her orders as to what to do when it came time to order cease treatment.
I couldn't do it. Because of the nature of the tumor only her kidneys were being looked at. A biopsy would have to be done on those which could have taken hours. And, because of a month in a coma, chances were, even her kidneys weren't usable.
The two ladies from the organ donor program were polite and consoling. But they were vultures circling the body, too.
I'm not trying to convince anyone not to be an organ donor. My point is, talk things through with your health care proxy (if you don't have one...get one!), examine all the scenarios you can think of, (and if you can't think of any talk to people who have been in the position before), put it in writing, and make sure you're clear on what your wishes are.