Good luck in Russian?

Желаю вам удачи! - zhelayu vam udachi! - to plural "you" or formal singular.
Желаю тебе удачи! - zhelayu tebye udachi! - to informal singular.
Удачи! - to either.
Ни пуха ни пера! - ni pukha ni pera - specifically when someone is setting off to do a task like a test or to ask a girl out at a bar. The appropriate response is к черту! - kchortu!