To get the best exchange rates, wait until arrival in the destination country, withdraw cash from an ATM using a credit or debit card, and pay by credit card whenever convenient. Check the bank and credit card fees associated with ATM withdrawals and currency exchange, and be aware of the going exchange rate before you leave to help make economical decisions.
Foreign banks usually offer lower rates than U.S. banks, so using ATMs during the trip and waiting until arrival to exchange cash saves significant money. Many banks and credit card companies charge a fee for each withdrawal from an ATM, as well as a percentage currency exchange fee, so it's best to make a few larger withdrawals rather than many smaller ones. Credit cards usually charge less than banks for currency exchange, so opt for credit over debit when possible.
If it is necessary to use cash only, exchange USD at a currency exchange in the foreign country rather than in the United States, and take the time to find the best rate. Stay up-to-date on the current exchange rate, and shop around, making sure the currency exchange does not offer an inflated "tourist" rate. Avoid going by the posted price; ask for the net rate after fees and commissions have been applied.