Look hot and be nice.
Make him say, "Damn, she looks good," and then when he expects you to be heinous, be nice, compliment him, say nice things to his current g/f if he has one. There's no better revenge than knowing you look awesome AND no one could possibly have anything bad to say about you. You don't want to be the girl that makes people go, "Well, she's cute, but what a b*tch."
As for outfits, I'd go with black pants or a long skirt, a nice sweater/top and a black peacoat. They're all staples of our sorority and would work for all of the events. I love black pants - they can be as dressy as you want them to be. Same with a nice peacoat.
