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.
Then there are the moments when something seems to be going wrong, when one or more areas of the business just don't seem to be working. Cash flow is anemic or the marketing message is flat. Perhaps customers have shown a marked interest in only one particular product or service, ignoring all your other offerings. This means it's time to revisit your business plan, more precisely it's time to revisit the questioning process that helped you craft your plan.