Formal:
* Veel succes en tot ziens! (This literally translates to "Much success and until seeing you!")
* Succes met alles en tot ziens! ("Success with everything and until seeing you!")
Informal:
* Succes en tot later! ("Success and until later!")
* Veel geluk en tot ziens! ("Much luck and until seeing you!")
* Heel veel succes! ("Very much success!")
* Tot ziens! (Just "Until seeing you!") is also common in informal contexts.
You can also use "Veel plezier!" ("Have fun!") or "Doe je best!" ("Do your best!") to wish someone good luck.
Ultimately, the best phrase depends on the situation and your relationship with the person.