Offer current customers a discount on next session's tuition for referred trial students that register for class. They will call you first. This should be the best return on your investment. Stick to your immediate area, no more than 10 miles depending on the profile of your typical customer and your area. Mail out birthday postcards to both active and inactive students.
Kids love to get mail.
Mail specials to inactive customers to get them back in the gym. Obviously, these mailings can be very inexpensive and easy to do provided you have a good computer program. Every community has a service that sends information on businesses in the area to new homeowners. This service is affordable and you get to the new customer first. Make sure you have an exclusive agreement to be the only gymnastics school.