Deciding on what to wear to a fetish party, largely depends on how you wish to be perceived and what your fetish is.
I once stood at a party sipping tea talking to a guy in a nappy. His only motive was he liked to wear a nappy and be looked at doing so!
Another will only wear rubber top to bottom and we converse in a muffled tone, its a job the hear through rubber!
What people wear gives a clue to who they are.
A submissive wears what the dominant tells her to wear, most often it will be little or nothing.
I like my subs to dress like a dirty whore so lingerie is very much in order with stockings and heels, whilst allowing some dignity, they are exposed.
I often chat to subs male and female who are just plain bollock naked.
No too subs are ever the same - each will dress according to the Dom's wishes, however i have always thought that lingerie and heels, certainly in the case of a female allow for a switch from sub to Dom and look good while doing so.
Most parties will have changing areas, so most will at least attend the party in jeans an t-shirt / casual and get changed on arrival.