Hi Sarah. Referrals from existing clients, friends, family, co-workers etc. I also think that writing is an EXCELLENT way to not only get your name out there, but also to attract new clients. With the internet, nowadays writing for publication (even your own blog) is easy. Also, network with other fitness professionals who are not necessarily in your same field (thus are NOT competitors); you’d be amazed how much cross-referral goes on within the industry.
I hope that this helps! Good luck!