If you’re heading to Japan and planning to use shinkansen (or maybe even the bog-standard train for more than a handful of journeys) then chances are buying yourself a JR Pass will be worthwhile.
You have to buy them outside Japan (which seems a bit harsh if you’re Japanese and wanting to travel around the country) and then exchange them when you want to use them. They can be used for 7, 14 or 21 days of unlimited travel across the JR network.
We bought ours via jrpass.com and the excellent Japan Guide has a full description of what they are and how to use them. They cost us £196 each but because the Yen is one of very few currencies that’s weaker than Sterling that’s actually gone down to £182 now.