The first thing anyone travelling to Japan should know about is the amazing JR Rail Pass.
Essentially, this is a pass valid for unlimited travel on all JR trains and buses, including high speed rail, Shinkansen. It is also valid for Miyajima ferry (which you will want to use, trust me). It can only be bought by tourists outside Japan. JR pass comes in a 1, 2 and 3 weeks variations and allows you to criss-cross Japan at a price of a domestic airfare.
There are some exceptions – the pass is not valid on the fastest train, Nozomi (it is only marginally faster) on the Tokyo-Hakata route but it is valid on Narita Express, the Narita airport – Tokyo link.
The best thing about is that it is not date-specific. You can activate it while in Japan on any date, whenever you are ready to take advantage of the world-famous Japanese train travel.
HOW TO BUY? Just go to the official JR Pass website for the list of authorised re-selellers. They will give you an exchange order that you will turn into an actual Pass once in Japan.
ARE ALL RESELLERS EQUAL? No. The prices can be vastly different. Make sure to call around for the best price.
THE BEST TIME TO BUY? Monitor the exchange rate. The price is published by Japan Railways in Yen. Note that not all resellers promptly re-price JR Passes. In Sydney, I found that AWL Pitt Travel have great, frequently adjusted prices.
WHERE CAN IT TAKE ME? In a nutshell: everywhere. Japan is amazingly well covered by the rail network. How about travelling from Tokyo to Hokkaido on a luxurious overnight Cassiopeia? (You will pay extra for the overnight berth). A day trip out of Osaka to Hiroshima? A special wide-view express through the Japanese Alps to reach Edo town of Takayama? Stay tuned for suggested itineraries.
FINALLY, HOW DO I START USING IT? That can be as easy as showing up at any major train station in Tokyo (ie Shinjuku), locating an appropriate Information Center or a Ticket Office, getting the magic pass into your hands and then showing it to the man at turnstiles who will wave you through. You won’t need to actually get tickets if you are using unreserved seats on Shinkansen. Just find the right platform.
You can also make the exchange ahead of travel and simply tell the Exchange Office the date from which you would like to start using the pass. ALSO, you can ‘buy’ advanced tickets by, once more, simply showing your pass. No money needs to change hands.
A WORD OF WARNING. You won’t be able to replace the pass if you lose it, so treat it as cash. Keep your passport handy as train conductors require to see your ID at the time of checking the pass.