When a new patient transferred into my practice, it was customary to ask why they chose to leave the last office.  The answers they gave might be very surprising to some professionals.

When people are unhappy with the practice, it is not necessarily you. 
Ask yourself- is there easy access to the office? i.e. parking available 
Is your staff friendly? 
Is the initial visit process intimidating or easy to slide through for the patient? 
Do callers get a live person on the phone after the first ring or two or do they have to go through ten automated button pushes until they are referred to someone ( a sure way to lose that initial caller) 

 Patients automatically refer friends and family when they are happy with the service, office, provider, practice. 

A mere thank you card for the one referring informing them that you recognize the special action they took and that you appreciate it also goes a long way. 

These questions may seem obvious but are not always addressed. 
If you make someone feel special and feel better, they will chat about it amongst their friends- the viral marketing is invaluable.