The best way to exchange currency is to simply withdraw cash from the ATM in the country you are visiting. Most American debit cards can be used in international ATMs to withdraw local currency. There will likely be fees of 3-8%, but some banks like Citi and Bank of America have international branches or partners that allow you to use your debit card for free.  You can also look for currency exchange branches within duty free shops in airports as commissions are generally less than other vendors or commissions are waived.    

You can also exchange currency at your local bank prior to departure. Banks tend to receive competitive exchange rates because they receive wholesale rates that aren't available to the general public. Just beware of delivery fees if you order cash over the phone or online to be delivered to your local branch - these fees can be as high as $10.

Websites such as oanda.com allow you to order cash online and have it securely shipped to your home, but beware of high delivery fees. If you are exchanging large sums of money, sometimes delivery fees can be waived if you ask. 

Traveler's checks can be purchased ahead of your trip and function just like U.S. dollars. Once you get to your destination, you'll need to find a venue that will exchange the traveler's checks for local currency which can lead to inconveniences. There are also associated fees as high as $9 per check. One of the best features of using a traveler's check is if they are lost or stolen they can be replaced usually within 24 hours, anywhere in the world. If you prefer not to use debit or credit cards or carry cash, traveler's checks are a good option. This may be a good option for travel to China as fees are low and the exchange rate is regulated by the government. 

Foreign exchange desks at airports, train stations and hotels are also an option, but commission/fees can be as high as twenty percent. You can exchange currency at your hotel as well, but exchange rates in hotels vary widely and generally speaking, the hotel rate will not be as competitive as rates outside of the hotel.. Currency exchange desks in city centers usually offer better rates than airports and hotels, but beware even those that advertise zero percent commission may contain hidden fees.

Using your credit card whenever possible is usually the best option. You can use your card just as if you are at home and the issuer will add a currency conversion fee of usually around 2-3% - you'll get the best exchange rate and lower fees than exchanging for cash. In many cases, credit companies waive the international transaction fee and provide the option to charge in USD or local currency.  Be sure to notify your credit card company of your travel plans before you depart, especially international travel, as many credit card companies track your purchasing patterns and may decline a charge if they see an international purchase where no pattern of international purchases exists.  It is not advisable to take cash advances from your credit card at an ATM. This transaction comes with fees as high as $20 per transaction along with any local ATM fees plus interest accruing on the advance starting the day the money is withdrawn.