Formal:
* Bis später! (This is the most common and neutral way to say it.)
Informal:
* Bis dann! (This is a slightly more casual way to say it.)
* Bis bald! (This means "see you soon" and is often used when you expect to see the person again soon.)
More specific:
* Bis morgen! (Until tomorrow)
* Bis nächste Woche! (Until next week)
* Bis zum nächsten Mal! (Until next time)
You can also use phrases like:
* Wir sehen uns später! (We'll see each other later!)
* Mach's gut! (Take care!)
The best choice will depend on the situation and your relationship with the person you are talking to.