Have you checked with the NPC office? They come out with a theme every year and offer T-shirts, graphics, etc. that coordinate.
http://www.greeklife.org
I've seen many cute themes:
Individually unique, together complete
Greek together, Greek Forever
No matter the letter, we're all Greek together
Also, check out my Panhellenic site for Rush Tips:
http://panhellenic.tripod.com
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Fraternally,
Barbara
Zeta Tau Alpha Alumna
If you have to go around telling everyone you're in charge you're not much of a leader.