I agree with what the others have already listed. I would like to add that word of mouth referrals are one of the best marketing tools you can have. Give 110% to every client and go above what is expected of you. If you already have clients, you can also ask for referrals (you can offer a free session or discount to your current client for any referral).
Also, networking in your community by participating in events can help get your name out. Sometimes you need to do this for free, but it’s worth your time. Lately I’ve been asked to donate a few sessions to some silent auctions for charity events locally. I always give a fitness basket with a few sessions to get my name out there (and I have gotten clients this way). Fitness events and health fairs are also good places to network, as well as local doctors and physical therapists.
For me it’s about making connections with people and being genuine. My approach with people is more educational and less sales. Never underestimate how you may have an impact on someone you meet!