[aephi alum]

IIRC, my chapter used a carrot-and-stick approach. People who were very active and involved, never missed chapter or ritual, etc. were recognized. Unexcused absences from the "biggies" (rush, ritual, chapter, and the big annual charity fundraiser) resulted in steep fines - I seem to recall that an unexcused absence from work week or rush was $100 per day, and this was ten years ago.

I'll point out that the "if you don't show you can't go to the mixer" threat won't work very well on a girl who is in a relationship. You might get an undesired response: "OK, I won't show, and I'll get out of the mixer, and I'll get two sorority-free evenings to spend with the boy!" Threatening to exclude people from formal has a little more bite - most people want to go to formal whether they're in a relationship or not.