Focus on Existing Markets
Here is an interesting list of Bootstrapping Lessons.
The first one is by far the most important:
Bootstrapping is about launching products on existing markets (so the idea is already validated)...Don't create new markets, don't chase innovative ideas. Prefer optimization over innovation.
If you are like me and you are not a fan of pre-validating ideas, this is important advice. Focus on existing markets. Focus where customers already exist (and are willing to spend money). Then find a way to make achieving success for them easier.