Having ACTIVE worksessions, drills, training. Doing so year-after.
Otherwise, on the bad side -- Creating a bureaucracy of "planners" who make plans in a vacuum -- that is, with little involvement or drills with the people and groups who would actually be involved does NO good.
For it is not the "plans" that matter -- what counts, what works, what is most effective in this kind of disaster is the training, the personal interlocking of associations developed in drills and worksessions, the command and field awareness of resources and procedures developed in those drills and work-sessions that enables a effective and marvelously adaptible response.
And that is exactly what Giuliani did. They had a plan and they had frequent, routine drills. It went a long way in maintining a sense of calm in the midst of utter destruction and chaos.