Check lists are important, but they are merely a collection of sometimes unrelated activities. A good plan has a logical flow. A good test of a plan is to hand it to someone with no knowledge of the event or organization and ask them to read it. If they can read it and understand the event and the reasons for the event, the plan is probably sound. Avoid the temptation to try to make it perfect the first time.
Planning is an iterative procedure.
Get something down on paper and refine it as the process goes along. Brevity is the soul of wit. The mind can only absorb what the rear end can endure. The initial meeting or discussion is the most important time in planning an event. Everything that follows will be refinement and addition of detail to the concepts developed in the initial meeting.