The future of your health is a matter of chance and likelihood: you have the power to shape that statistical landscape through good lifestyle choices and strategies such as helping to fund research into rejuvenation biotechnology and signing up with a cryonics provider - but nothing is a certainty. You can shift your chances, shift your life expectancy (itself a statistical measure), but you can't entirely remove happenstance and sheer bad luck. You are far better off by making and following good plans, but bad end results are still possible.
For example, even someone who signs up to be cryopreserved and does a good job of managing the organization of his own cryosuspension at the end of life can still be cut short by bad luck:
Alcor member A-1088, Dennis Ross, was pronounced legally dead on Sunday October 30, 2011. A neurocryopreservation, Mr. Ross became Alcor's 108th patient. Alcor received emergency notification that a member in the St. Petersburg, Florida area had been rushed to the hospital on Friday, October 28th and was diagnosed with a massive intracerebral hemorrhagic stroke due to a ruptured brain aneurysm.
Suspended Animation (SA) went to the hospital and began to prepare for a probable cryonics case. Through medical imaging on October 30, physicians determined the individual's brain damage was so extensive they declared him brain-dead. After the family decided to withdraw life support, SA performed field stabilization and attempted washout; however their success was limited due to the compromised blood flow of the brain. SA completed a neuroseparation before shipping the anatomical donation on dry ice to Alcor.
It's important to recognize that, despite best reasonable efforts, the possibility remains that we are going to be betrayed by our own biology in the end. The quote above is an unfortunate example of the type, in which the patient suffered a brain-damaging end of life incident that will greatly reduce the possibility of a good cryopreservation - and that despite high quality support from medical staff and everyone else involved in organizing the response. You'll recall that the point of cryonics is to preserve the fine structure of the brain, within which is the data that makes up the mind - keep that and none of the other damage matters in the long term. But the more neural damage that occurs prior to cooling, the worse the end result will be.
So the best preparation in the world can be sabotaged by the body breaking down in exactly the wrong way at the end of life. All we can do is strive to minimize the risks. In the case of cryonics, many of these late stage risks exist because Western legal systems make it impossible for people and services to collaborate in order to arrange the time and manner of death. Self-determination in end of life choices, and the ability to help people enact those choices, would make cryopreservation of the old and frail far less expensive and far less subject to risk of neural damage. This is just one of countless injustices and losses of freedom inherent in modern governance and law.
But back to the original point: the best we can do is to work to minimize risk. Risk cannot be removed entirely, and we all live in fragile bodies.