Ask what led to the referral
Next time you see the client who sent you the referral, ask about the circumstances that led to it. It leads your client to relive the situation, and makes it more likely they will make another referral in the same situation. Knowing what the client said will give you valuable information about what the client values and how to teach other clients to do the same. It gives you a chance to confirm that they understand your value proposition and teaches you how they describe it. It enables you to discover a new trigger phrases that prompted your client to refer.
Source: Stephen Wershing for SEI's "Practically Speaking" blog