I am constantly flip-flopping on my opinion of this. I completed a living will on Monday. I never want my family to have to make this choice, or to have to possibly fight it out in the courts.
For anyone in the US, you may find paperwork for your state at the following site. For my Maryland the link is to the Maryland Attorney General's office, where free forms are available.
http://uslivingwillregistry.com/forms.shtmDonnaP99