nothing, but then you really arent following the rules cause you have to be wearing something they can't believe, so i think eastcoastjibber has the best answer, they will be expecting a costume, so wear regular clothes. if not, then wear ski stuff
but for what you should wear, if you want total shock value, corset, bondage belt, and tiger striped bastard pants. or, just wear jeans and have your nuts hanging out over the top
me and my buddies found like 100 pairs of slacks and had a pants party. we made everyone that came wear them all night and let me tell you, it was out of this world..
actually wearing normal clothes for a "i can't believe you wore that to this party" theme is like the best thing to pick. because your supposed to wear something off the wall, but when u come in wearing normal clothes, everyone will be like "i can't believe you wore normal clothes to this party"