At every meeting that I have with the prospect on their road to becoming a client, I talk about them introducing me to their friends, and I usually have about six meetings before I am finished with them.
(You may have as few as two meetings with a new client. The idea is to mention the issue of referrals each time, without any expectations.)
I may say something like, “Do you have any friends that might be able to use this type of service, or is this something that somebody that you know might be interested in?”