Many times we let the location help set the theme for us. Examples the event space above the baseball stadium - diamonds are a girls best friend, or semi-formal in a mexican restaurant - dirty dancing havana nights. If your not so lucky to have it jump out at you (like having it in an hotel) go classic. We had Famous Couples as a mixer, not sure how it would go in formal dress (???) but I'm sure y'all pulled it off. Or my favorite just go look at shirt designs and find one your in love with and plan it around that!
For shirts try
www.prographicssportswear.com, they do all of ours and they always look great, have tons of designs, super friendly, and cheap.