Spread It Out
When traveling in a foreign country, especially for extended periods of time, it is important to spread your money out. You don't want to keep everything in a wallet because if your wallet is lost or stolen, you'll be out of luck. Take with you a money belt, which will allow you to store extra cash. Place emergency funds in your backpack, as well.
Instead of bringing hundreds of dollars of cash with you, bring different credit and debit cards. These will be much safer to travel with. All ATMs in Brazil will accept your cards, allow you to withdraw money from your account and will convert the money to reals, the Brazilian currency. Withdraw only what you need for a day or two. So if you lose your cash, your entire trip won't be ruined; you'll just have to go easy on the spending for a few days.
Bringing gift ATM cards is a good option. These cards, which you can purchase online, have a set amount of money on them. Spread several of them out, putting some in a wallet, others in a pocket and so on. If someone steals one or more of them, you won't be out your valuable credit or debit card. Because these cards are not from a bank but a company, there generally isn't an ATM transaction fee, which most bank and credit card companies charge for overseas use.
Use the ATM
Brazil has many different banks you can visit to withdraw money or to take cash advances from with your credit card, but it is better to skip the teller window and head for the ATM. This is because bank and credit card fraud are commonplace in Brazil. Avoiding any kind of unnecessary contact with your plastic will limit the chances of being a victim of these crimes. It is also another reason to bring the disposable gift ATM cards; they contain none of your personal information.
In addition to your cash, carry some traveler's checks. The American Express brand is the most widely accepted brand, and you can have your money refunded if it is lost or stolen. If you use traveler's checks, take them to a bank; banks will give a much better exchange rate than money changing booths.
It is recommended to bypass the money changers and go directly to an ATM. They will often give much better rates than money changers. If you have extra cash you want to change to reals, however, feel free to use the money changer. Count your cash when you receive it. More reputable establishments will give you a receipt with your exchange.