Announce the meeting adequately in advance. Give jobs to as many people around as you can. Getting people involved keeps their commitment level high. Stick to agendas, start meetings on time, and try to anticipate the next step beyond the one you've already scheduled. Distribute thanks widely to everyone who's helped, in any way.
Appreciation is a valued currency in the volunteer world.
Avoid violating any laws or rules within your school. Although freedom of assembly and thought are part of the American fabric, students must obey certain codes, too. Failure to comply could bring penalties or even shut you down. Notice your own emotions. Anger and extreme joy are often a part of the organizing experience in advocacy.