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"In preparing for battle I have always found that plans are useless, but planning is indispensable." - Eisenhower

The idea being that the process of planning makes you more easily able to react when everything goes to shit. Eg don't treat a plan like a recipe for success.

[herewulf]

Another fine military planning quote: "A plan only lasts until the first shot."

[tdfx]

And Mike's Tyson's derivative: "Everyone has a plan until they get punched in the face." Make sure your plan is flexible!