Where, exactly, are you going to find your customers?

IMO, this is the single most important part of the entire process and if you can answer this question you are way ahead of the game.

But you should be able to provide answers for this question very early in the planning stage.

If your answer is from the search engines, what is the basis for your answer? Are you good at SEO? Have you done any keyword research?

Planning on posting at forums in that niche? Are there any with decent traffic AND allow sigs or at least profiles? You better check.

Thinking about PPCs like AdWords? How much are your bids going to be? And home much traffic can you expect? Do you have experience with PPCs?

The answers to this simple question will decide your sucess. It is really that simple. But like playing an old blues song, it's simple but it isn't easy.