I hear this question a lot. Unfortunately, there's not a simple answer. In my case, it was a matter of blogging for years, building up a modest, but interested, audience, having about 2,000 Twitter followers at the time, and spending about $5,000 on advertising once we had been live for about 3 months. I also blogged extensively about the process of designing the application and some of the other steps involved.

Really, though, it's not the first customers that are hard. It's maintaining and continuing to work on it and grow it during the time where growth isn't amazing and it isn't yet a full-time job.