Formal:
* Bonne chance! (Most common and versatile)
* Je vous souhaite bonne chance. (I wish you good luck.)
* Tous mes vœux de succès. (All my wishes for success.)
Informal:
* Bonne chance! (This is used in both formal and informal contexts.)
* Bien joué! (Good luck, literally "play well" - often used in sports.)
* Allez! (Go! - used to encourage someone)
* On croise les doigts! (We cross our fingers!)
* Je t'embrasse! (Literally "I kiss you" - used as a way to wish someone luck)
Specific situations:
* Bonne chance pour ton examen! (Good luck on your exam!)
* Bonne chance pour ton interview! (Good luck with your interview!)
* Bonne chance pour ton spectacle! (Good luck with your performance!)
Important note: "Bonne chance" can be used in almost any situation, so it's a safe bet if you're not sure which phrase to use.