The Seishun Juhachi Kippu is now on sale at all JR ticket offices - be sure to take advantage of this, traveling everywhere on the local trains for ¥2300/day.
If you are booking one of the night trains with the Seishun Juhachi Kippu, I should mention I had a little trouble booking the Moonlight Nagara route from Tokyo to Ogaki. Apparently you need to be very, very specific when making this reservation; in other words, you need to know the stops. You have to book three different tickets for this route: Tokyo (東京) to Odawara (小田原), Odawara to Nagoya (名古屋), and Nagoya to Ogaki (大垣). It may vary from station to station, but the staff didn't understand me when I said I wanted to go "Tokyo kara Ogaki made" (from Tokyo to Ogaki). Eventually it was sorted out. The fact that the day changes when you're on board the train doesn't help either; if you're using the Seishun Juhachi Kippu, pay about ¥500 for the first leg (the last few minutes of the previous day), then use your Seishun Juhachi Kippu for the next day. This site has good information about the seating options.
Looking for winter travel plans? I'm checking into the Sapporo Snow Festival (Sapporo Yuki Matsuri さっぽろ雪まつり), running February 6th-12th. This is one of the largest festivals in Japan, the biggest in Sapporo, and besides... who doesn't love snow? Come for the ice sculptures, stay for the 3 AM Ramen. It's a good life. You might want to book quickly - air and hotel reservations start opening up two months prior.
You might find these links useful:
Sapporo International Youth Hostel
Japan Guide Forum
Domestic Travel in Japan
Toyoko-Inn (The only place I found with any rooms available for Saturday night)
If you are making domestic travel plans, you might want to avoid using HIS Travel. Japan Probe had an interesting article about them regarding fare hikes for foreigners. Read it here.