When your ventures in Japan brings you outside of any wifi hotspots, it's a great thing being able to stay connected without having to resort to insanely expensive roaming charges. What you do is rent a pocket sized LTE wifi router, connect your devices to it, and make sure it's with you whenever you happen to be out and about.
I usually go with Telecom Square. The have booths at the major international airports, rental plans are decent and it's all really convenient and easy. You simply walk up to the counter and ask to rent a portable wifi router. Sort out the formalities, return it at one of the major airports when leaving Japan, and that's all there is to it. And you can preorder one to be waiting for you if you don't want to risk them being out of units.
Another decent option is Japan Wireless. You order online, pay upfront and then they'll ship the router to your hotel. A return envelope is provided and when it's time to leave Japan, simply put the router into a post box and you're done.
Lastly, it's a good idea to keep a rechargable extra battery around to juice up the router (or your smartphone) through USB should the need arise. Those batteries are available in a bazillion different shapes and sizes at most japanese electronics stores, even Muji has a few nice and simple ones.
Last update July 20th, 2017. Originally posted May 7th, 2013.