Ask them questions, about their preferences and how they are going to realize them, until any internal incoherence is revealed. For example (roughly), ask them if they care about their health and if they want to see the future. At some point they will have to draw a line between what they are already doing to survive and cryonics. Ask why they draw that line...

Questioning is sometimes better than preaching because it causes the people who are questioned to reveal their own ignorance or incoherence while it allows those who ask the questions to act submissively.