We are all familiar with the postmortem. When things go wrong, we want to understand why they went wrong so hopefully we can avoid them in the future. This works well for classes of failures that are due to things like infrastructure or procedural deficiencies. Maybe you had a missing alert in your monitoring system or something wasn't communicated to the right person. The problem is that we can't plan for the unexpected and it's the unexpected that causes the most problems.
The way the premortem works is this: imagine it is one year from now and (insert your project here) has failed horribly. Please write a brief history explaining what went wrong.
What this attempts to do is to bypass our natural tendency to downplay potential issues. Whether you are in favor of the project or not, this exercise will engage your imagination to come up with failure scenarios you might not have otherwise considered.
Give it a shot and please post comments about what you thought of it.