Hi and welcome! Our chapter didn't use Bid Day shirts, but rather Rush t-shirts, which we also wore on Bid Day.
One Fall we chose a Ben & Jerry's ice cream theme and wore shirts with "Hofstra's Phinest" across the front. Another Spring, a majority of the girls were into Broadway, so we used the musical, "RENT." We wore black shirts with A, E and Phi in block letters (like the show's logo) and on the back was written, "No Other Path, No Other Way, No Day but Today, Rush AEPhi." However, this was before the show was scheduled to close.
Also, I was at the mall this weekend and thought it may be cute to use "Abercrombie & Fitch" as in "Alpha Epsilon & Phi." Maybe written out with "New York, Est. 1909" styled after the A&F line. Using a giraffe instead of a moose or something of that nature? Just thinking out loud, I guess! There are also some super cute ideas here:
http://www.greek101.com/screenprinting/designs.php
Good luck!