File this between your ACLU card and your AAA card.*
Available in "pubs, banks, libraries, GP surgeries, even some churches, the Advanced Decision to Refuse Treatment (ADRT) card sits snugly in a wallet or purse and instructs a doctor to withhold treatment should the carrier lose the capacity to make decisions, because of an accident or illness."
Oddly, these cards are being offered by a U.K. city council, but private groups offer the same service in the U.S.
* OK, that would be a pretty weird wallet, since the card is only available in the U.K. But you get the idea.