I went to Yamasa in Okazaki, Aichi Prefecture, for about 3 months a year and a half ago, and it made a huge difference in my Japanese ability. Going to school 6-7 hours a day, in a full-on Japanese environment, needing it to eat, drink, etc., is a great way to boost language skills in a short period of time.
Also, it's Okazaki, so you're not going to be able to "English" your way out of most situations. (Japanese was the only way I could communicate with some of my classmates, who didn't speak English, and it was the only way to communicate with the teachers.)
I highly recommend that school, but any good language school will do. It's great for concentrated language learning, IMO.
But if you can't afford to go there and live there for some period of time, then there are alternatives.
For the last few weeks, I've been trying this to help with N1 prep and generally keep my Japanese levels in shape, and keep my language knowledge growing: (someone recommended it here, thanks, whoever it was)
http://www.japonin.com/
I like the Flex classes so far, and the price is pretty good (if you pay in Yen, and not in Dollars. Their dollar pricing is way out of date. 円安万歳!)
The way the courses are taught, you can just 'drop in' at any point you want to.
The classes are voice chat + text chat + a powerpoint/whiteboard viewer + video (optional). The teachers even take notes for you in the chat box, so you can just download it all to a text file when the class finishes. So far, my classes have been small. About 3 people per class. The cap is 7 people.
You can check the syllabus for the content any lesson offered ahead of time; the texts for advanced /intermediate are Kanzen Master 1/2級 grammar books, or the N1/N2 versions; and the teachers so far have been very helpful.
I'd say that the general level of teaching is roughly about the same as Yamasa. (I'm taking advanced/N1 classes. No idea how it would be at lower levels.) I did the 3-lesson trial, and went ahead and bought some more lessons. It's easier than driving an hour each way to the nearest place here in NC, it's cheaper than going back to Japan, and I can do lessons whenever they fit into my schedule as long as I have Internet.
I will have to think about how I deal with the material both before and after classes, though. There's some new stuff to consider, like "How do I put this in my Anki deck?" "How do I keep from forgetting this?" etc.
Do the 3-lesson trial first, to see if you like it.
EDIT: Just after I say that, they went and changed their Dollar prices:
http://www.japonin.com/doll_yen.html
Still, I saved some money paying in Yen with Paypal, FWIW. Use XE to figure it out if you're going to buy lessons.
Edited: 2013-05-18, 2:06 pm