I think I can speak to this pretty well, as I helped
rescue my chapter of Kappa, raising numbers from four to seventeen in one year. Panhel helped us a bit, and we're still here.
The first thing to realize is that low numbers are usually not the problem, but rather the most visible symptom of problems within the organizational structures of the chapter and Panhel. The chapter needs to solicit help from its IHQ and alumnae, get itself turned around internally, and make a plan to get back on track. Secondly, Panhel needs to realize that a gross imbalance in chapter sizes is symptomatic of problems within Panhel. Maybe the dirty rushing is out of control. Maybe there's lots of trash-talking going on. Maybe chapters aren't using release figures properly. They need to work to fix these flaws and get the system running smoothly. I'm not saying that all the chapters have to be the exact same size, but when you have six chapters at 100, a chapter at 60, and a chapter at 40, that's indicative of Panhellenic problems. Panhel needs to realize that if the smallest chapter dies off of campus, someone else is going to be on the bottom. It's not healthy for the Greek system at all.
What the sororities did for us: They quit trash-talking, spreading rumours and gossip, and worked toward Greek unity. The other chapters on our campus actually didn't COB for a term so that we could. The Rho Chis encouraged women to give us a second look during Recruitment. Panhel pushed our COBs and gave PNMs our phone numbers. They helped us put up signs for our COBs and other events. They co-sponsored events with us and put our name first on every poster. And they sat back and let us shine for a while. They knew that doing these things wasn't going to hurt them in Recruitment-- sure enough, they both took quota and then some-- but that it'd increase the number of women going through Recruitment.
Something that hurt: "Oh, go back to Kappa, they need people." The others gave off the impression that we were taking anyone and everyone, and some people were turned off because they felt that Kappa was too "easy."
What the fraternities did for us: They quit trash-talking, spreading rumours and gossip, and worked toward Greek unity. They were falling over each other to do mixers and philanthropy events with us. They doted on our pledges. They gave us a round of applause at the all-Greek party. They talked us up to freshman guys around campus, who in turn spread it to the freshman girls.
It can be done. It's hard, but it can be done.