The best time to consider a living will is when you're in a calm frame of mind and if possible, in reasonably good health. Also, keep in mind the case of Karen Ann Quinlan: she was only 21 years old when she fell into a coma. The cause of Terri Schiavo's coma is still unknown [ref]. In other words, there's no specific age or state of health that marks when you should decide on a living will and health care proxy. Unfortunately serious medical events can happen at almost any time and to anyone.
Before drafting a living will, you should talk with your doctor, family and friends. Share your feelings and opinions with them so that they are informed about your thoughts. Being open and communicative about this process will help in case a situation does arise where you may need to use your living will. Your friends and family will know to look at the living will and that you put time and consideration into your decisions. Conflict among family members may also be prevented if they are aware of your wishes.
It's also important to consider how this process may affect your friends and family emotionally and even financially. In addition to being costly, long term care for someone who is incapacitated or otherwise beyond recovery is an intense, draining process.
Senior centers can provide help, advice and reference documents, and some non-profit organizations or clinics can as well. Be wary of living will seminars that charge a fee -- senior centers and hospitals often provide the same service for free. Pre-made forms are available, but it's important to tailor them to your needs.
An attorney is a key partner in drafting a living will. He or she will be able to advise you on its legal implications and to adapt the necessary forms so that they conform to state law.
If you spend significant time in more than one state, make sure your living will applies in both states. Though hiring a lawyer isn't required, one who is experienced drafting wills can be helpful for answering such questions.
After your living will is complete, give a copy to your doctor so that it can be put into your medical file. It may be wise to keep a card on you or to wear a bracelet stating that you have a living will on file. You should also give a copy of your living will to your health care proxy (more on health care proxies below) and make sure that he or she has a clear knowledge of your wishes.