To build a customer database, keep a guestbook at your front counter, and encourage visitors to sign it. Have a space for them to add their name, phone number, email address, and ages of potential students. Enter this information into you school contact database and use it for mass emailing. Keep copies of your latest newsletter and class schedule at the counter so visitors may see what you may offer them.
Consider hosting events (not just demonstrations). Have snacks, music, and lots of hands-on exhibits. Find out which nonprofit organizations attract people who might also be interested in the martial arts or other individual, physical sports, and then donate school branded martial art objects to the charities' silent auctions or gift bags. Form alliances with noncompetitive businesses where your customers spend money, like restaurants or health clubs. You can share your client databases (assuming you tell customers in advance and allow them to opt out) or display their business brochures in your school. Doing this spreads the cost of marketing.