Place a premium on collaboration. More can be achieved working together. Assign tasks in pairs to have members help each other. Set the stage for open-mindedness and a broad range of viewpoints to make the group a comfortable setting for all members to share. Set goals as an entire group.
People own what they help create.
Take steps to keep your own motivation high. This will positively affect the motivation of those around you. Go to a thrift store and buy several really terrible ties, then give them as appreciation gifts to all of your members. After the meeting, you might convince everyone to put the ties on and head out on the town. Put a classified ad in your student newspaper every week, thanking a member who has made a contribution to the organization.