For a general "cheer up"
* Kia kaha! This literally translates to "be strong!" and is a common phrase used to encourage someone.
* Kia pai! This means "have a good time" or "be happy."
For a more specific "don't be sad"
* Kaua e pouri. This directly translates to "don't be sad" and is a more specific way to express concern about someone's mood.
For a friendlier "don't worry"
* Kaua e manukanuka. This means "don't worry" and is a good option for a casual conversation.
Ultimately, the best way to say "cheer up" in Māori depends on the situation and your relationship with the person you're speaking to.