Death is an inevitable part of life. You will never know what will happen next. For this reason, there is no excuse to fail to plan for your end-of-life care. In the case when you are in a permanently unconscious state but legally alive, you won't be able to make decisions.
A living will is put in place to dictate what will happen with your assets and how your current medical providers should treat you in the event you are unable to make your decisions known.
A medical power of attorney (or health care proxy) allows you to appoint a person you trust as your health care agent. He/she will be authorized to make medical decisions on your behalf. 
There are certain cases where it is legal for an individual to not honor your living will.  For example, emergency medical technicians cannot honor living wills or medical powers of attorney. Once emergency personnel has been called, they must do what is necessary to stabilize a person for transfer to a hospital. Only after which can a physician evaluate the person’s condition and determine whether their advance directive can be implemented.
No matter where you are in the US, you can have an advance directive, as it is legal and available nationwide. The most noteworthy is that laws governing the process vary from state to state.  So check with your legal advisors about how you want it to be done. Advance directives don’t cover every state.  It is advised that you complete your advance directives on a state that you plan to spend a significant amount of time in.
Advance directives do not expire.  The effect will remain the same unless you change it or until you complete another one, which invalidates the previous. It’s advised that, if you wish to change the details of your directive, you should complete a new one instead.
Thinking about planning for your end-of-life decisions can be scary. However, it can certainly reduce the burden on your family and can somehow improve their quality of life after you are gone. Make sure your last wishes will be fulfilled, plan your advance directives now.
1. Goebel, Karen, and Mary Therese Crave. Advance Directives for Health Care. learningstore.uwex.edu/Assets/pdfs/B3604.pdf.
2. American Bar Association. Patient Self-Determination Act: State Law Guide. American Bar Association Commission on Legal Problems of the Elderly. August 1991.
3. "Health Care Proxy - New York State Department of Health" (PDF). New York State Department of Health. Archived from the original (PDF) on 2014-06-11.
4. “Exercise Your Right: Put Your Healthcare Decisions in Writing.” MedStar Harbor Hospital, ct1.medstarhealth.org/content/uploads/sites/7/2014/09/MHH-Advance-Directives.pdf.