The best marketing activity, especially for a service business like yours, is word-of-mouth from your current customers. Not only is it low cost, but it is much more likely to result in a “sale” for you. If a friend or family member of mine gives a strong recommendation for a product or a service to me, I am much more likely to look into that than if I noticed a Facebook post, a paid listing, etc.
So how can you encourage this?
The first thing to do is to ask!
Talk to your clients, send them an email, call them on the phone, hand them a promotional flyer … encourage them to help you.
You could also consider something more tangible. For example, one free class for every new customer they refer to you. Or maybe a new exercise mat (with your logo on it, of course).
This requires you to have good systems in place, both to know who you current customers are, who are the new customers coming on-board, and how they heard about you.
All the best.