- Run a Contest on your website, say you will provide them with some fancy gadget for the chosen one among the correctly answered.
- Send a mailer to your list of clients and ask them to refer more.
- Ask them some related questions?
Provide a hint button and link to your website content.
The hint button leads to the content of your website (it could articles, white paper, or just plain news)
here is a link to one such contest.