Well, with my choral group it's the regulation long black dress, black hose, black flats, no perfume, no jewelry, etc. My conductor is apparently much like _gargoyle's; I have no wish whatsoever to face her wrath.
On the few occasions when I've performed less formally, it's been clean jeans and a decent shirt. For casual performances, I figure it's mostly a matter of personal taste and the audience one is playing for.