Given that they found that most people are less supportive of ending treatment for others than they are for themselves (they are more likely to think they'd want their own plug pulled than to be prepared to pull the plug on a loved one) the living will is a vital way of expressing your personal wishes on end-of-life issues. There are all sorts of resources available on the Internet, and you should seriously look into it.
The document is important, but it's also key to regularly discuss your feelings with whoever would make that choice. Crazy things happen, and you want them to feel confident that they are acting in your best interests. Because without guidance, they're likely to keep you hooked up to machines well past the point you'd want to be.
While we're on the topic, the same goes for organ donation. Just checking the box on the back of your drivers license is not sufficient to be a donor. Your family must know what you want also, because they'd have to sign the relevant papers. Again, discuss it regularly.
A few years back, a friend was working at the Transplant Olympics, an annual event that does just what it sounds like. He met a mother whose daughter had died in a car crash. She went down to Atlanta to hear her child's heart beat again, and to see its recipient compete. Absolute magic. He cried about this woman's story when he told it weeks later.
There are few things we can do that will more profoundly affect other people than organ donation, so give it some thought. There's more information available in the Red Cross link in the sidebar.