Who likes to think about what will happen if you or your loved one can no longer make medical decisions? If you say you do, you're lying. No one really wants to contemplate the idea of themselves or their loved ones being incapable of making their own decisions. It's scary and uncomfortable. Because of this discomfort, though, we can neglect making the arrangements for the kind of treatment and care we would want at the end of our lives.
I found it very helpful to keep a signed Advance Directive, and not just at the end when my husband couldn't make his own medical decisions. It was handy to have whenever I had to pick up copies of my husband's medical records, because it meant I didn't need to have him fill out a separate form giving his permission. Medical professionals will generally choose to withhold protected information if they are not absolutely certain you have the patient's consent to know. I also found that hospitals and some medical practices asked for it every time my husband was admitted.
You can get an Advance Directive form from any hospital and most doctors' offices for free. If you are a California resident, here is a link to a free Advance Directive form from the California Attorney General's website.
Follow the instructions, sign the form, and have it properly witnessed. Give a copy of it to your primary care physician and always carry a copy of it with you.