In addition to a will or trust, your estate plan should also include a power of attorney (POA), which gives someone else the power to act on your behalf in the event that you cannot do so yourself. Since this person will be allowed to make important legal decisions as if they were you, be sure you trust them and that they understand clearly what your wishes are.
There are two primary types of POA:
Durable power of attorney: a person or organization of your choosing can make financial and legal decisions regarding your property.
Healthcare power of attorney: a designated person is allowed to make all health care decisions for you if you are unable, including whether to keep you on life support.