A laminated card listing twelve requirements that have never once appeared in a single human being at the same time. Ten years of experience in a tool that has existed for four. Fluent in three languages. Available immediately. Salary marked competitive. On the reverse, a blank space, for writing down the role that actually exists once everyone has agreed to stop describing the unicorn.
Keep beside the screen while writing the spec. Refer to the reverse early and often.
Part of The Control Room, for those who can't see what's working.