Avoid the invisible ghetto at all costs. This is not specific to Japan, this applies to all situations where individuals go and live in another country where another language is spoken:

It is extremely easy to fall into the trap of unconsciously and pretty much by default surrounding yourself with English-speaking people and spending most of your day speaking English. Why? Because suddenly you notice that you have so much in common with that one English-speaking person who –wow!– is from the States too. What are the odds! Interacting and bonding like that with every or any given person who can speak English in the US would be ridiculous. But abroad, you suddenly end up spending most of your days speaking English to your new English-speaking BFFs and then you maybe even share a flat and so on and so on. That self-ghettoisation happens almost by default, because by nature humans have a preference for the familiar. You really have to make an active effort to resist that and create exposure to and surround yourself with Japanese. Yes, even in Japan that can take active effort to do.