Philanthropy events are a great way to go. A couple of years back members from another sorority on campus came to our rescue when a huge fraternity party got rescheduled for the same evening as our philanthropy event. We were bummed because we were having a hard time getting guys (the usual target for philanthropy events) to sign up and when some friends on EC of the other sorority heard what happened they encouraged their members to get involved and offered incentives for them to sign up and attend our event.
We ended up having a great turn out and it was a huge success. I was talking to one of my friends in the other house about it later after telling her how grateful we were and her response spoke volumes:
"Sure some things get competitive and recruitment can get catty but philanthropy is something that can be respected and supported by members of all organizations across the board - No matter the letter we're all greek together"<- I know she's cheesy, but i love her for it