For those who have built and deployed applications before, how do you go about planning/developing your idea before you begin building it? For example, do you sketch out rough drafts with something like Balsamiq Mockups or just pen and paper? Do you bother filling out the YC application to flesh out your idea (even if you don't plan on applying)?

There are different approaches to achieve this, but finding a technique that is both comprehensive enough and time efficient is the challenge.