I have been using Steve Pavlina’s advice on business plans for indie game developers. I have found it to be a good guide, even though I think it is geared towards existing developers. There isn’t much talk about what to do if you’re just starting and you don’t have a product yet.
Luckily, Juuso Hietalahti recently wrote The Basic Marketing Plan for Indie Games. Even though it isn’t as detailed, it is a good supplement to Pavlina’s document.