Formal:
* Gracias y adiós. (pronounced: gra-thee-as ee ah-dee-ohs)
Informal:
* Gracias y hasta luego. (pronounced: gra-thee-as ee ah-sta loo-eh-go)
You can also say:
* Gracias, hasta pronto. (pronounced: gra-thee-as, ah-sta prohn-toh) - which means "Thank you, see you soon"
* Gracias, nos vemos. (pronounced: gra-thee-as, nos ve-mos) - which means "Thank you, see you"
The choice between formal and informal depends on the situation and your relationship with the person you're talking to.