Erica, what my chapter does for Pref is when we go hand out the invites (no this is not violating rules, we do not have a Panhell) the invite says something like:
"The sisters of Gamma Phi Beta cordially extend an invitation to our Preference Night. This is a serious evening where the ladies will be sharing a very special side of their sisterhood with you. Attire is formal, please dress accordingly."
That usually works - although we once had a girl show up in khakis and a fleece.
