Involve members in goal setting and decision making. Clarify your expectations of members and their expectations of you. Occasionally serve food at your meetings. Have contests and give small prizes to the person who designs the best program, etc. Use team building activities to re-energize the group and strengthen loyalty and commitment.
Motivation comes from inner needs, drives and goals.
As a leader, your task in motivating others is to tap into these and supply a channel for their fulfillment. The individual members must do the rest .