I would suggest creating a business page on facebook and inviting all your friends to like it, then post fitness related material (videos, quotes, photos, when your fitness classes are if you teach any, any discounts you may have, articles, etc) weekly. You can do the same thing with twitter. If you have an email list, then monthly newsletters can be useful (mailchimp is free & easy to use). Get the clients that you’ve trained to review you on yelp (I’ve gotten a lot of clients that way). Last but not least, keep showing up at social events and meeting people.