I've wondered about this. In my mid-20s I decided upon a signal that I would send myself should I be stricken with a mental illness like Alzheimer's. I don't know if it'll work, but I remember it very strongly and wonder if I will be able to recall it should such a situation arise. I'm in my mid-30s now and have hopefully a long time to go before I should start letting people in on my plan.

The idea is that if I am aware that I am suffering from dementia or Alzheimer's, I will hopefully make life easier for those caring for me (e.g., stay calm when my son/daughter I do not recognize is attempting to care for me).