Wow.. I don't know what else to say after all the great comments.
Advertise like crazy! If your campus has a university radio station, try going there to advertise events for Recruitment/ or COB events (I used to be a DJ at our station, and the station was really good about advertising campus organization events for free) Try also university newspaper announcements... thanking groups for helping with events, advertising events, community/university service projects.. keep your name out there!
If any of your sisters are in other organizations, it's also a great way to recruit quality new members! Tell them about your sorority and invite them to recruitment/cob events!
I hope this helped
Best of luck with your chapter