One easy way would be to make organ donation opt out instead of opt in. As in, if you do nothing, you're an organ donor if you end up brain-dead, and if you don't want to, you have to explicitly opt out. Alternatively, we could just say any brain-dead person may have their organs harvested, regardless of what they declared while still alive. After all, you don't need the organs anymore once you're brain-dead. (I'm specifying brain-dead, because if you're completely dead, then the organs are also useless)