First and foremost you must overdeliver your services and produce results in your attendees so that they spread the word. They will become your walking-talking billboard for referrals. This is the best free marketing you can ask for.
Then there are many other ways to fill the spots:
Bring a friend day
Referral specials such as for every person you refer you will receive free classes
Online newsletters sent to your list with useful information to keep people hooked
Hope this helps