Okay just thought I'd clear up a few things based on some experiences...
Regarding how much effort will be put into saving your life if you are donor: Total myth. I work in hospitals. Organs are useless if you're dead. There are two sources of donation: Donation after neurological death (brain dead, but heart still beating); and donation after cardiocirculatory death (you're in an ICU, and they take you off of life support even if you're not brain dead, but they know you won't survive without life support - your heart will stop). Either way, the window to procure the organs is very small (usually 1-2 hours). So yes, every effort will be made to save your life, and you will not be removed off of life support without your family giving consent. So talk to your families as someone else said.
Regarding the comments on walking past the homeless: My family's business was a group home for alcoholics. These alcoholics were most of the "homeless" in the city the business was in. They'd go out for an hour, beg, finish off with $50 in their pockets, and then go buy booze. So no, I don't give to the homeless either since that money will mostly go to booze or drugs. I also knew a guy that was living off of disability, but would be a beggar during business hours. Made a ******** of money and drove a nice car. I contribute with my taxes, and there are programs available to the truly homeless. I will buy a coffee if someone asks outside a coffee shop, I will give food, and I might slip a twenty if I see someone lying on a subway exhaust at 2am in minus 20. But the average "homeless" person during the day I will walk past as well. Does that make me less "worthy" to receive organs? I've dedicated my life to helping others. I'm on a Board of Directors for a Shelter, and I work in hospitals, and I helped with the family business looking after alcoholics. You don't know who it is that will walk past homeless, and why they do it.