Some time ago I wrote an article on a Living Will, extolling the virtues of something called Five Wishes. I still think that it is an excellent document as it allows one to describe, for many different circumstances, what you want done with your life when you are unable to make the choices yourself (or where family or medical community think you are unable). Unfortunately Living Wills, I have recently learned, are not binding in Massachusetts.
Instead there is a Massachusetts Health Care Proxy, easily Googled.
Requiring two witnesses, it sets up one or two names of agents who can speak for you. Although there is no extra place on the form, I would suggest adding a line saying something like: “details of my wishes, to aid my agents, can be found in my Living Will”.
Here is some addition reference material from the Massachusetts Tax Law Library: http://www.lawlib.state.ma.us/subject/about/healthproxy.html