I'm going to get exceedingly practical today.
There are things that every person of legal age can do that can help you get the care you want and need, and keep you safe from the interventions you neither need nor want.
For starters, study and pray. Learn what it is, in your depths, that you do want. Listen to serious discussions of medical ethics, such as those taking place right now that have been instigated by Terri's situation. Read some of the fine teaching documents that our Pope and our bishops have issued about the end of life. A particularly good and easy to read one is Now and at the Hour of Our Death, which the bishops of my state wrote several years ago.
Then, talk about it. Have serious conversations with your family and your friends about medical care, about disability, and about end of life issues. Not just a "by the way" reaction to some passing news item or TV drama, but actual conversation, discussion, even argument, about what and where and when and how. Hash out just what it is that you want and that you believe, and make sure that those who care about you, and those who are legally connected to you, like spouses and parents and sibs, know what they are and, if possible, how you got to them. Even if they don't agree with you --- in fact, ESPECIALLY if they don't agree!
You are NOT too young or too healthy to consider or talk about these things. Elderly will inevitably arrive, and Disabled is the one minority group that anyone can join at any time, involuntarily and without warning. If you are old enough to read my blog, you are old enough to start studying and thinking about and discussing these things.
Next, write down in a legal kind of way instructions for when you cannot give instructions, in one of the ways recognised by your country or state. In my state, there are two ways to do this, the Declaration to Physicians [pdf file], also called a "living will", and the Durable Power of Attorney for Health Care [also pdf file].
Generally, Living Wills are a bad option. They by their nature define things too strictly and they do not establish anyone to make the judgment calls that pretty inevitably must be made. Then, by law, they must be followed even if the circumstances change. Also, most Living Will forms work from the presumption that one would rather not live. [Though some right-to-life groups have developed living will forms which support life support, such as this one from Wisconsin Right to Life.] However, if there are certain things that you know ABSOLUTELY that you want to keep from happening to you --- an example might be a Jehovah's Witness who absolutely doesn't want to be transfused. or someone who absolutely refuses any consent for surgery --- then the Living Will may be for you.
More generally useful, however, is the Durable Power of Attorney for Health Care. This document appoints one or two persons to make decisions for you when that time comes that you cannot do it for yourself, and gives some guidance and restrictions to them. Choose people that you trust, and who agree with you about end of life issues and the last things. If there are particular things you want or don't want, or particular instructions that your proxy needs to know, put them down. Mine states that if I need to be permanently institutionalized, that it be in a geographic region where I can be regularly visited by my relatives, and also states that continuation or discontinuation of life support should be only in accord with Catholic moral teaching. I chose two of my siblings who are most in agreement with me about both the Faith and the stage when it is time to let go. Choose people you trust. If that isn't your spouse or your parent and they think they ought to be, have that fight right now, while you can still stick up for yourself! And, at least once a year, read your Durable Power of Attorney and make sure it still reflects what you want and that you still trust the ones you chose --- and void it and make a new one if that is indicated. Don't let it moulder. [Your beloved fiance of 1998 could well be your abusive alienated ex in 2007 when you have that auto accident......] Keep it up to date, and make sure all your doctors have copies.
This is how you keep yourself from being the center of a legal tug-of-war when you become disabled or are approaching death
And, while you are thinking of end things, how about some basic funeral planning, making sure your kin knows where your plot is if you have one, maybe some basic instructions? And, a regular old will, so your children have a guardian you would want and your kinfolk don't fight over your stuff the morning after the burial .......