Alcor does a good job at their website clearing up the major myths and answering questions about cryonics, the practice of low-temperature, ice-free storage of the body and brain after clinical death. After you die, the structure of your brain remains intact - if stored, plausible future technology could one day restore you to life. The important thing, the core of what is you, is the data represented by the preserved structure of your brain.
But if you want to benefit from cryonics, you have a little work to do. Like most forms of insurance, it has to be set up in advance and preparations kept up to date if you want to benefit. Over at cryonics blog Depressed Metabolism, Aschwin de Wolf looks at some of the self-sabotaging assumptions that get in the way:
Unfortunately, advances in the science of cryopreservation will not automatically translate into better patient care. Other factors, such as the delay between time of "death" and start of procedures, and the protocols, equipment and personnel of the responding cryonics organizations, matter as well. For example, if a cryonics standby team is not able to get to a patient before 24 hours after cardiac arrest, pumps him full of air during remote blood washout, and ships him back to the cryonics organization at subzero temperatures, that patient will not benefit from advances in human cryopreservation such as rapid induction of hypothermia, neuroprotection and vitrification.
Both critics and supporters have made specific probability estimates about how likely cryonics is to work. In its worst form such probability assessments convey nothing more than putting a number on overall feelings of pessimism or optimism.
At the time a person really needs cryonics, he may no longer be able to communicate those desires, lack funding to make arrangements, or encounter hostile relatives. A more subtle variant concerns the person who expects that aging will be solved before cryonics will be necessary. This person may or may not be right, but such optimism may not make him more immune to accidents than other people. This mindset is often observed among young "transhumanists" and practicing life extensionists.