To get the best exchange rate, wait until you arrive at your destination. While most major airports feature a currency exchange desk, you are likely to get a better rate directly from an ATM machine affiliated with a major bank.
ATM cards most likely to work trouble-free overseas are those with a four-digit PIN number. Since you may be charged a usage fee by both the local bank and your home institution, it's advisable make one large instead of several small withdrawals whenever possible — and keep your cash in a safe place out of pickpockets' range.