ATM's: These are common enough in the major cities. Often they are located in an isolated room, air-conditioned, with glass walls. Every machine I encountered had at least English and Spanish as language options. The fee is most commonly $3, and a withdrawal limit is seldom under $500. The ATM's are all connected to banks as opposed to independently owned and operated machines.

Traveler's Checks: Are likely a waste of time. Panama doesn't have the same proclivity to cater to tourists that many countries do, and it seems most of this country runs more on cash than on banking. An advertisement for accepting traveler's checks is virtually unseen; to exchange them you would need to go to a bank which would entail finding a bank. - On finding a bank, you will be regularly told that 'no we do not accpet them but this bank does.' On arriving at the said bank they will tell you the exact same.

Credit Cards: The more remote you are, the less likely your credit card will be of any use to you. In Panama City, not every store is set up to take them, though this is where you'll find them most helpful. Some of the hostels in the city even are cash only. Outside of Panama City, Even at places where you are spending at least $50 you may not be able to pay by credit card. 

Cash: Cash is your Friend in Panama! The currency is the Balboa: the paper bills are the U.S. dollar, and the coins used here are equally the U.S. Coins and the Balboa coins, which are nearly identical in size and weight, and only different in the image and print on the coin. Avoid large bills- even a $20 can be difficult to get change for at times, if you need it in somewhere like a taxi. The smaller the bill the better, not only for convenience, but also when you are negotiating a price. 

Personal Checks: Leave them at Home.