I have to agree with what everyone said here & reiterate.....the best way to get women to pledge is to get your non-Greek friends interested! My chapter received some of our best sisters this way. Also, befriending a group of independent girls is another good idea. Get one interested....tell her to bring her friends....they'll (naturally!) fall in love with your chapter and pledge as a group.
A note on advertising.....yes, mucho fliers/chalk is kind of desperate looking. Rock your letters around campus and just make everyone aware that you are an XYZ. The best advertisement is you!
What types of events do you have planned so far?
Good luck!!!!!!!