To build a consistent and strong chain of referrals, one must build massive TRUST with clients. You must be trustworthy enough for them to recommend you to others and even trustworthy enough to have new clients brought to you directly. You must learn to invest in your relationships with clients and create a healthy relationship built on trust.
It is not enough that you get the job done, but it is necessary to do the job well, do it consistently and never fail to bring value.
Trust = value = strong client relationship = referrals