Personal invites are the way to go.
If the chapter at my school had done that, maybe I would've been a DZ a year earlier!
The DZ chapter at the university I went to recolonized, and they were in their first year as a colony when I was a freshman. There were about 3 DZs on my dorm floor, and I remember seeing all the COB signs they put up in the bathroom and thinking that maybe I'd go. But I always lost my nerve! I was petrified of showing up at the house and ringing the doorbell and...well, I'm not exactly sure
what I was afraid of, but I can definitely understand why girls might be reluctant to show up for open houses cold turkey. I never in a million years thought I'd end up in DZ when I started rush the next fall, but I think it's funny that I
almost went to sooooooo many COB events but always chickened out and I ended up there anyway.
So what we used to do with COB was, each member would take a couple of invitations to an event--pumpkin carving, TV night, whatever--and hand them to girls they knew from the dorm, from class, etc. It really did make more people show up, and members got to sort of pick who came so that many times, the PNMs actually ended up joining.