[lasertron]

I'm not a "you have to have a business plan" person at ALL, but working on one for my own startup helped me IMMENSELY in communicating the idea, the value proposition, how we are going to gain users, and how we're going to scale. Because I spent so many hours refining that document--just for my own benefit--I can speak about the inner workings of the business more confidently. It also gave me the chance to anticipate problems and address them before investors or customers could teach me about them the hard way. Obviously, a good business plan (or "business planning document," or "cool-ass outline," whatever verbiage you want to use to describe it) won't save you from a ton of hard lessons to be surmounted, but it gets your head in the game from the onset if you begin with one.

Besides the normal mapping and purpose you should outline, it helps to think of a list of objectives and then detail meaningful, practical ways you are going to achieve those. That's where you ask that investor for help, and bring all the data and numbers you can.