Formal:
* Deixe estar: This is the most common and neutral way to say "never mind." It literally means "let it be."
* Não se preocupe: This means "don't worry" and is more appropriate if you're trying to reassure someone.
Informal:
* Esquece: This is a more casual way to say "never mind" and literally means "forget it."
* Tanto faz: This means "it doesn't matter" and is good for situations where you're not particularly bothered by something.
* Deixa pra lá: This is another casual way to say "never mind" and literally means "leave it there."
Example:
* A: Você quer um pedaço de bolo? (Do you want a piece of cake?)
* B: Não, obrigado. Deixa estar. (No, thank you. Never mind.)
The best way to choose the right phrase depends on the situation and your relationship with the person you're talking to.