A smooth tongue
Before leaving on vacation, I suggest you listen to some Brazilian singers such as Gilberto Gil, Chico Buarque or Maria Gadu. It'll help you get an ear for the language and the difference with European Portuguese.
If your Spanish is okay, then you should be able to understand Brazilians and, with a bit of effort, they will probably understand you. For everyone else, here are few useful words and phrases to help you during your trip to Brazil.