Domain expertise and talking to users.

By working with the users you want to sell to or even just the industry, you will see how they do things - and more importantly, see what it takes to influence their behavior.

You may realize pain points you didn't see in the first place - and may even end up rethinking your parameters/format of downloadable as well. It's a huge commitment, however, so most people do the second, which is talking to users.

Read Paul Graham's essay on launching.

Startups are a marathon, not a catapult. You are looking for consistent, incremental progress and growth over the lifetime of the company, not one short-term rocket-ship blastoff.