If I were you, I'd start by talking to your chapter advisor. She may have some suggestions, and she can put you in touch with people on the regional or national level who can help.
Have you already had fall formal recruitment, or do you have deferred recruitment?
Be careful with the postering, as it can come across as desperation. Women are more likely to respond to a personal invitation, so ask each sister to think of 1 or 2 unaffiliated women and personally invite them to your rush events.
Wear letters around campus, and be visible...
Good luck.
