Who are your current customers, and why do they buy from you? Look for common characteristics and interests. Which ones bring in the most business? It is very likely that other people like them could also benefit from your product/service.

Here is where some guerilla tactics come in handy. Use Google Ad Planner to look at some competing websites to identify demographics that are specific to their audience.

Then visit their Facebook and Linked In company pages (if they have them). Find out what kind of people "like" your competitors. Look for trends in gender, interest, hobbies, and "likes".

Finally, go to Google Keyword Tool and plug in your competitor sites. The tool will spit out a list of search terms people might find the site with (this list is great for finding out what people are truly looking for).