Randy Komisar's book Getting to Plan B is an excellent read and he has some very useful guidelines and advice to the entrepreneur.
Plan A is Necessary but not Sufficient
Plan A is useful for a number of reasons... The process of creating Plan A is useful in and of itself. It forces you to have to confront your assumptions. It forces you to have to model out your challenges. It forces you to have to deal with mitigating your risks. It also allows you to start to create a language that you can share with people in the business community, the language of the business plan, of revenues, of profitability, of growth, of customers and customer value and pricing. This language is very important and starts to translate your ideas into a business. So, it is important to have a plan and its important because it allows you call out the assumptions and the risks. And that is where the getting to plan b process starts.
Analogues and Apologues
What things are like (in other successful or failed ventures). An apologue is akin to an allegory, that is an extended analogue.
Note that Komisar says antalogues in the video below, but I can't seem to find that word as an actual word. The idea is to see what didn't work as well as what did work. This avoids the problem known as confirmation bias, that is only looking at the winners. This is a basic problem in the very popular book Good to Great.
Leaps of Faith
What is unknown, clearly focus on three of these. Make sure they are the most important three to the business model and organization success. These are good because they focus the entrepreneur on the things that will ensure success or result in failure.
Feedback from Customers
Cannot ask customers directly in terms of what and how to innovate. Instead pull together contextual information. Things like ATMs failed in focus groups.