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.
I have been putting together business plans for over 25 years and it is clear to me that the strength of its core rests solely on being able to execute the plan. Each year I approach business planning as an opportunity, rather than a burden. I would rather invest the time up front in mapping out the upcoming year, than leaving it to chance to dictate my strategy. While this may force me to think strategically as well as tactically, preparing a detailed business plan in advance enables me to identify the challenges in advance of actually facing them.