The NPC total here is 65. We are also a commuter school, but not as small. I can throw in my 2 cents.
There was a "recruitment" advice page on the local sororities site that I thought gave some good advice for any sorority in general.
http://www.geocities.com/localsororities
Do you all have formal recruitment there? I imagine it may be difficult with small numbers. So if you do COB type events in the fall, what about doing something really "different." Like bowling or some other off the wall activity. (Laser tag comes to mind... I don't know why.) Doing something fun and advertising the be-jezus out of it seems to get a lot of girls.