My school also has a small Greek system, so we advertised for formal (Panhellenic) and informal (just us). We advertised on Facebook and by the bookstore. That's the main place that any organization/event/cause hands out fliers on our campus.
__________________
On the heart of each sister
lies one 0-----,, that binds us
What's campus housing like? Do you guys have all-female dorms?
A great way to promote your sororities is to help out with dorm move-in days (with permission from the powers that be, of course). Wear your letters proudly while you help these girls get acquainted with their new campus home. They get introduced to Greek Life ASAP. Hopefully that will generate some interest!