What should I know about throwing a surprise masquerade party for my wife's 30th birthday?

She’s dropped the hint about wanting to have one a million times (I think she watched Labyrinth a few too many times as a kid) so I’m going to do it and do it well. I’m going to try and get a nice fancy ballroom and all that stuff. There crucial thing is keeping it secret which I’m a bit unsure of – I’m guessing that I’ll have to do the entire organization myself so that nobody is aware. However, what is a good tactic for getting invitations out to people in such a way that they won’t spill the beans? There will be a fairly large number of people (~100) so the likelihood that one of them spoils it is relatively high. I’ve never organized anything like this so any pointers are appreciated.