It will help if you have an advanced directive/living will. I have one that says that in the event that I'm in a coma, I want no measures taken (feeding tube, ventilator, etc.) beyond comfort care (pain meds). However, I believe this only covers a coma or severe brain damage (like Terry Schiavo). If it's physical damage, you're still screwed.
Edit: Also, an advanced directive is not a DNR. They will still revive you when you are found, but will pull the plug when it's determined your brain is fried. I'm going to attempt to get a DNR, but am unsure my doc will go along with it. I have several health problems, but I'm only in my 40s. I'm trying to put together a convinsive argument, like my frail body wouldn't do well with chest compressions (I have osteoporosis), and stories of a friend and relative who were revived, only to have their families pull the plug a couple week later (one was a head injury, the other was a weird adverse reaction to a new med).