Optimizing for a Small Niche
Answer: Track what you are ranking for and look for meaningful patterns and descriptive related keywords in that data.
If few people are searching for something, but you are in a growing field, then that might be great market timing which helps makes you a market leader. Make sure you create things like an industry glossary, and actively participate in communities related to your topic so you have top of mind awareness to people in related fields. Research why people are linking at competitors, and what is missing in the marketplace.
If you are already in an established field, then use keyword tools to look for broader keywords or parallel keywords. Observe what descriptive modifiers people are using for higher volume keywords. The odds are pretty good people will use those same type of modifiers when searching for your topic. You can also use this cast a wider net concept by bidding for related broader keywords on AdWords and track the referrals.