well.... i hope i can help you out mate
i went to japan first time last year for 4 months, from Oct '06 - Jan '07 and i had a GREAT time
so the season's were late autum to mid-winter (i guess) and i was combining travelling with snowboarding, the latter being the last part of my trip...
things i did before i went:
crammed on the JP101
did a lot of internet research
and brought the Lonely Planet (backpackers travel guide)
what i didnt know about was the " Japan Rail Pass " a friend who lives in Japan told me about it.
The 'JR Pass' was really REALLY great... its only available to foreigners, ie tourists, and you can only apply for it IF you are NOT going to Japan to work.
So see your local Travel Agent an tell them to get on the case.... it comes in a (memory dont fail me now) 3 week pass, and i think 2 week pass, costing around 30,000 - 40,000 Yen, but dont quote me on that as i cant remember rightly, however i got 1 of each (the 3 week pass being more expensive)
The pass entitle's you to unlimited travel on the Shinkansen and the normal trains from the date that you activate it (from a JR desk) so if you are going to travel lots around Japan then this is a wayyyyyy cheaper way of doing it as i found. (I travelled a lot during the first 2 months) also you can get Reserved Seats so you dont have to cram an hope for a seat in the Unreserved Cars.
I stayed in backpackers throughout the journey an also has the few nights in Hotels to spoil myself....
The Lonely Planet was great in this department of locating backpackers.
BEWARE, some backpackers have curfews an if your not back by the time they lock their doors, have a good sleep outside hahaha jus hope its not winter
Actually mate, if you want any more info just drop me a message here, or in my e mail i guess an i would be happy to share my experiences with you
I went back again in April, and again in June (this is HOT as its Japanese Summer) and will go again in December of this year and again in Feb of next year
Its a great place mate, an i hope that you enjoy it as much as i did an continue to do.
Stay safe