Formal:
* Bonne chance: This is the most common and versatile phrase, suitable for most situations.
* Je vous souhaite bonne chance: This is a more polite and formal way to say "I wish you good luck."
Informal:
* Bonne chance! This is a shorter, more casual version of "bonne chance."
* T'as de la chance! (literally "You have luck!") This is a very informal phrase, and it's best to only use it with close friends.
* Tout le meilleur! (literally "All the best!") This is a good choice for wishing someone good luck in a more general sense, like for a new job or a big event.
You can also add a specific detail to your wish, like:
* Bonne chance pour ton examen! (Good luck for your exam!)
* Bonne chance pour ton entretien d'embauche! (Good luck for your job interview!)
* Bonne chance pour ton déménagement! (Good luck for your move!)
Remember that French speakers often use gestures and facial expressions to express their good wishes as well.