Everybody recommends using the ATMS.... but no one so far has pointed out that it makes a BIG difference what bank card you are using. If you draw from accounts at the "biggie banks" (Chase, BankAmerica, Citi, Wells Fargo, TD etc), you may get "sticker shock." The Greek ATMs charge NOTHING for their use, but your home bank may! If you have a Plain Vanilla card at one of the "biggie banks" you will be charged $5 for each transaction and 3% of Amount Withdrawn. Thus, getting $300 worth of Euros will cost $5 + $9 = $14. Highway robbery. Why?? Because they Can. Only reason. And if you use a Plain Vanilla CC from these robbers, you'll pay 3% of the amount charged. Again, just because they can. if you have an "elite" card (based on $$$$ in account, or paying annual fee) this may differ, but I'm talking about Ordinary folks. Save the CC for important purchases you want back-up on, i.e. jewelry.
To save on just getting cash, you can go 2 ways. (1) open an account online (Capital One etc) just for your travel use -- these usually charge NO fees or % or (2) if there's time before your trip, open an account at a Credit Union, these usually have minimal fee, like $2 & no %. There are now credit unions in most states u can just join, don't have to work for a special place. Google your state.
It is ALSO important to notify the issuing institutions for your ATM and your Credit Cards that you'll be using the card abroad. Othewise the first time you try to use it, they'll freeze the account as protection against fraud. Also, one rookie mistake is to tell the Bank you'll be using the card in Greece. Then on the way over, you change planes in Frankfurt, and decide to get some Euros there... and whoops! Account locks up. Just tell the bank "abroad" and give the dates.