Make it a contest among your board and current members to sign up one or more new members each meeting. Offer incentives for this such as a gift card or free admission to future events that would require payment. Be sure to make personal attempts to reach past members that have not joined yet for this year. A phone call is always a good option or even a hand written letter. Recognizing these members on an individual level goes a long way for retention.
Offer classroom incentives so the teacher can help you recruit families, use business cards and have current members pass them out with their info and the date of the next meeting to make it an invitation to check out what your unit has to offer. Keep using social media to promote the events your unit has hosted for the students along with a Join Now link.