Existing clients are the most obvio

Existing clients are the most obvious source of referrals--but, of course, you have to get some first. When I was still in massage school doing practice massages in the student clinic, I met and massaged some people who are still my clients today--a dozen years later. They've sent many referrals my way. The key to getting those word-of-mouth recommendations is not just dependent on technique; it's dependent on giving great service, starting appointments on time, giving clients what they're paying for, and presenting a professional persona. You're providing an overall experience clients think so highly of they'll tell other people. Cultivate that from day one.