Remember the proverbial expression 'not seeing the wood for the trees'? You need to see the 'wood' first, then delve in and start examining the individual 'trees', meaning the individual items which you will be breaking down later. So a great point is to make sure that you have that overarching vision _ and if you cannot find one, then maybe it is an indication that you are obsessing on a few technical aspects that do not necessarily make up a whole business as you had imagined it. A business that makes sense and is going to be sustainable in the future is one that has that clear vision within which all the smaller parts contribute to make it successful.
Fundamentals, Cycles & Trends (FC & T's): Your plan, if done in advance and thoroughly, should provide and excellent foundation from which to work. Even the best plan still has to react to outside forces that will influence your best intentions. Identifying certain fundamentals, cycles and trends that may impact your company is a prudent way to being able to develop a contingency "plan B" in the event an outside force rears its head. A series of key FC & T's should be monitored throughout the year so that if required, your plan can react. Certain FC & T's may include wholesale pricing, weather, commodity markets or labor market impacts that are out of your control. In my opinion, developing contingency plans in advance for these outside forces at least gives you a fighting chance to react favorably.