ATM's might be the easiest way, but I still think that travelers cheques are the cheapest in most cases.

You can exchange TC's at the airport for a 0.5% commission rate, whereas they charge 2.5% for cash. So, if you are able to get USD TC's for no commission or fee before you leave, your total conversion cost will be 0.5%.

Although it's possible to beat this with the right ATM card, the vast majority of US-issued ATM cards will charge at least 1% in foreign currency charges, plus in many cases a network ATM fee.

Note that this advice is for Japan only: in most countries, TC's are more expensive to redeem than cash or most ATM fees.