I haven't been to a black tie event since I was, oh, 20-something? I own nothing even remotely posh frock-ish and for one night I'm thinking I'll probably rent. But what? What the hell do you wear nowadays? The kind of 'ball dress' I wore 15 years ago is not going to suit me any more. I'm 38, 5'4", slim, but hourglass shaped. Any ideas?