American credit and debit cards are accepted ALMOST NOWHERE in Berlin. The only feasible option is to take out lots of cash from ATMs whenever you can. The problem is, ATMs are not so common in Berlin either.
There is one located at Sparkasse at Heinrich Heine Str. Before you leave the US, speak to your bank about (1) the high fees they may charge for overseas use (perhaps they have a partnership with a German bank) and (2) not flagging your account as fraudulent due to the overseas use.
To counter the large fees some students took out large amounts of cash at one time. If you choose to do that be careful with your cash. (Watch out for pickpockets!)
Many places do not accept credit cards.
Those that do more widely accepted MasterCard and Visa over AMEX.
Some students came with them as backup for their cash and credit cards.
Do not rely on them as they are not always accepted.
If you lose them, or if they get stolen, you can put in a claim for your money.