Sorry I should have mentioned - anyone who has been to Japan knows that credit/bank/travelex cards aren't very widely accepted by ATMs as Tubby Beaver correctly pointed out.
HOWEVER - if you go to the post office all of the above cards will be accepted and work well, offering you better exchange rates than any other money exchange.
It's simple really - get out $1000 at a time (make sure you increase your daily withdrawal limit at the bank) and see how long it lasts. If you're going to Niseko note that you will need to take a bus to Kutchan to go to the post office to perform the abov