One thing I'm not seeing mentioned here, and I think it's worth noting, is that if you put together any document like a DNR or a living will, it must be notarized before it can be considered legally viable. Just writing "do not resuscitate" on a piece of paper (over-simplifying but you get my point) is not going to mean anything if anybody tries to dispute it.

Best bet, go through a site like LegalZoom to get it put together if you can't afford a lawyer for a will-writing session, and make sure it's all wrapped up with a notary so you don't have to worry about someone making a decision on your behalf that you don't want.