How much cash you want to carry on you will vary with the country size and amount of time you plan on staying .
Generally , work from a maximum allowed bank machine withdrawal down until you have about two days of money- burn left , then find a bank machine again.
Carry only a small amount in your user wallet, maybe barely enough for one day. Leave the bulk stash in a safe place like a money belt in your undies. Restock your daily wallet only in absolute privacy . Don't give street pickpockets a clue as to where you carry it .

If crossing e. g . Mexico or Brazil over several weeks it is not advisable to get all the money in cash . Visit bank machines to get it in reasonable instalments.
If you are ripping across a tiny country in a day or two it makes sense and you might be no worse off if you got it at a border money changer by trading in the currency you will not be needing anymore. It is going to cost some regardless of where you convert , but try to avoid blatant ripoff exchange rates.

As Mollydog suggests , have several cc and several bank cards which work for different handling systems . Look on the backs of the cards and cover as many bases as possible with logos for Maestro, Cirrus, Mastecard, Interac, Plus, Interlink , Visa and others.
The more options the better for hitting a match to bank machine connections .

Big savings to be made in Argentina if you have us dollars and can make use of the dolarblue exchange rate

Km

Charles Scwab debit card does not charge fees and will reimburse foreign bank charges.

For Brazil, try to avoid the ATM's from "Banco24Horas" as they charge transaction fee to boot. The effective conversion rate fom them is also lousy. Try to use the ATM's from the major banks as Banco do Brasil, Banco Bradeco, Banco Itaú, Caixa Econômica Federal etc.

Santander (Mexico) and Bank of America
 

As mentioned in this thread, check with you credit card companies and bank to see who has the best deal. In my experience, going back as early as 2008, Santander Serfin in Mexico is a partner with Bank of America. You can withdraw cash from a Santander ATM with your Bank of America ATM card, zero fees, nada. In some of the big tourist area you have your choice of withdrawing USD or MX pesos. Before opening a BofA account, check to make sure the partnership is still in effect. PS: Santander banks in other countries don't have the same deal with Bank of America. Sames goes for HSBC, Scotia, CitiBank...they are all franchises, without relationships that cross international borders.

I recently heard you can get better than bank rates when exchanging USD privately in Brazil. Venezuela and Argentina are great places to get great exchange rates when trading on the "private" market. In both of these countries I transfer funds from my Bank of America account to a friend's Bank of America account, then got local funds from them. Trading USD with street money changers in Lima (Nov 2014) saved me bank fees and got me a good rate. Hard to exchange outside big bills once I got outside of Lima.

Download a currency conversion app for your smart phone. I use the free app "Currency" for my iPhone. A conversion app will help you strike a reasonable deal at the border and informs you what price is being asked in your home currency.

In the end, get a credit card that cuts a fair deal, then deal with the fees. Its the price of adventure...and its worth it.

Great stuff!
Just a quick note on using street money changers or casa de cambio's. Trust me, they will check your $100 USD notes carefully ... so you need to do the same with their big bills. There is a TON of counterfeit money all over south america ... and they don't do a good job of catching the crooks. :oops2:

Try to have a known to be good bill (get from legit bank) and compare to bills the money changer gives you.

Lots of Gringos have been ripped off this way so do be aware. The locals can spot the fakes quickly ... so asking a local friend for help can also work (it's what I did)

Peru
 

In November 2015 I met a young backpacker in Lima, Peru. She got stuck with counterfeit bills...somewhere.

A week or so later I was in gift shop up in the mountains buying a handful of stickers for everyone on the tour...and the shopkeeper put a thin piece of paper over the Peruvian currency I was paying with. She took a pencil and rubbed over a section of the bill. Their currency has special printing, so the pattern sticks out when this rubbing trick is applied. My money passed the test. It makes me wonder if shopkeeper look harder at the money travelers pay with, because we are more likely to be the victims and accept funny money.

PS: I had a taken a small stack of $100 bills when heading to Peru to join friends on a commercial tour. Before leaving Lima I wanted to exchange USD for Peruvian soles. When I ask a bank guard if that bank exchanges currency, he took out to the street and pointed to a cambo guy. I got a better exchange rate from this guy than the bank and no fees. Your mileage may vary.

Barclay
 

Barclay Card World Elite Arrival+ First year membership is free. No int'l transaction fees.