[dirtyroboto]

For any short, slow moving financial period you will never make any profit as the banks have that covered.
Even at the best buy rate and the best sell rate you will loose out.

The only exception is if there is a massive change in value while you have the dominant currency.

There is no money to be made by the casual traveler, you can only try to find the best rate. As many have pointed out, Japan does not have the best rates. Most if not all Japanese banks charge a massive fee that is calculated on the % of the held currency in relation to it's rate.

The best way to make your money go further is to use a direct debt card. You pay a rate set by the bank in your own country. Just hit a participating ATM in any country and you should get a fair rate.
Caution on this, as it is better to withdraw the limit, not just a few bucks as the transaction fee will be the same.