An advance directive says what you would want done about your medical care if you were unable to make a decision at that moment, such as being in a coma. A psychiatric advance directive states what you would want done if you were unable to make a decision about your mental health care. You can either limit it to what you write on the paper, or you can name someone (and a backup) who can make these decisions for you.
Different states have different laws regarding advance directives and psychiatric advance directives. The National Resource Center on Psychiatric Advance Directives is a good place to start looking for information in your state.
It can be difficult and quite painful to think about a scenario in which things are so bad you cannot legally make your own psychiatric treatment decisions, but I think it is worth that difficulty to guarantee that if something like that does happen, you still have a say in your care.
I recently completed my psychiatric advance directive. In it I have listed medications that have been helpful in the past, and medications I refuse to take, such as habit forming benzodiazapines. In addition I list what hospital I would prefer to be admitted to. I also list both a primary and secondary decision maker. The last and probably the slightly more rare written decision was that any major medication changes must first be run through my current psychiatrist.
What I wrote on my psychiatric advance directive is just an example of what you can decide with it. I think anyone who is living with a mental illness should consider completing a psychiatric advance directive.