My chapter once did a thing where you got X% off your dues for every girl you COB'ed...
However, the topic of this thread was not what chapters can do internally, but what the system can do. So I will steer it back to that.
Release figures are critical. I agree with everyone who said that already.
Panhellenic should run ads in the paper after rush begins. I heard about this at MGCA, and really pushed for it on my campus, and I know it worked in at least a few cases. Something like "Missed out on rush but still interested in a sorority? E-mail
panhellenicrushchair@uiuc.edu for more info." The VP recruitment then forwards the info to the chapters. This allows chapters to reach girls who may be interested without the stigma of advertising themselves. Also, if you have a long rush, and this is done as soon as the first round is over, girls can meet the chapters and get their bid on bid day with the rest of the NM's (if your local rules allow).