Do you send out mailers to all incoming freshman and unaffiliated returning students?
Make sure to post flyers, banners on campus.
Station a recruitment sign-up desk at dorms, cafeterias.
That's pretty much what we do here... there's a place where you can go sign up for recruitment, booklets with a section for all the sororities (pictures, general info goes in those) and then the booklets are sent out to all incoming students (including transfers).
Fliers, sidewalk chalking (some campuses care about that, others done)... things of that nature. We aways have Tshirts advertising formal recruitment, too....
Have a nice BIG ad in your school newspaper ... preferably near the horoscopes or the comics. Post flyers in dorm cafeterias, the union, mcdonald's, etc. (anywhere that there's high traffic). And have your e-mail address available to answer questions.
We have a bulletin board at my school that we decorate with general chapter info and pictures, as well as with recruitment info. We also have tables at all the freshman orientations where PNM's can ask questions and get information.