Formal:
* Best wishes
* Good luck
* Every success
* My best regards
* I wish you well
Informal:
* Take care
* Have a great time
* Keep in touch
* Let me know how it goes
* Cheers!
More specific:
* Best of luck with your new job!
* Have a fantastic vacation!
* Congratulations on your wedding!
* All the best in your studies!
The best phrase to use depends on the relationship you have with the person and the situation.