Referrals are always going to be the best form of lead. However no matter what you can never really rely on getting them consistently from any client.
One way to at least help you ensure that you do get them more consistently than not is to talk about referrals before you actually close the deal.
"Joe, one thing I always like to talk to people about before I work for them is how much I value being introduced to others so I can help them (add in here what you do/are going to do for the client), WHEN you are completely satisfied with my skills and the work I've done would you be willing to introduce me to 2 other business owners you know who would benefit from the same service?"
Then go a step further and include it in any contracts you have.
This will help you to have a more steady flow of referrals.