E.C.
Volunteering in the classroom is the best way to start IMO - it gives you an idea of how things work in the school and it also gives the teachers, administrators, and other active parents a chance to become familiar with you so that when an opening *does* come up and you apply, the folks at the school who are part of the hiring process will give you a closer look b/c they know you're already a dependable volunteer. As a regular volunteer, you're also more likely to be informed of openings before they get posted than someone who isn't around the school much. (I say this from experience - this is how I ended up getting a job at my girls' school)