- Break a leg!
- Ни пуха, ни пера! (a Russian good luck charm, literally wishing a hunter to bring back "neither fur nor feather".)
- Dire lui merde! (French, lit. "tell him shit" = "wish him luck").
I vaguely remember that the French example was originally a wish for a huge wealthy audience arriving in horse carriages - thus, a lot of horse shit as a sign of artistic success.
As for the other two, I guess, the idea is not scaring away (not jinxing?) the good luck. Any more cultures sharing this superstition?