Personally, I just listen to the "all conversation" tracks every week, and then read Japanese literature or play video games. Basically, replace whatever english language materials you have access to with Japanese ones. For instance, my computer has Japanese as the default language, as does my phone.
Also play around and see just how much Japanese you need to be doing daily to not forget stuff. If you're somebody like コッドさん who has deadlines for exams, you might want to go over more text books, otherwise you could probably just get by with some light reading.
As to Japanese at university, there may be some clubs at your school or in your region that deal with Japanese language/culture, so perhaps you could use those as a substitute for the interim.
Also, a lot of universities are affiliated with ones in Japan, and tend to have exchange research programs, so if you're not up for MEXT (as I am; I'd have to fly overseas to take the exams both for MEXT and JLPT), you can always look at what programs universities offer. Asian Language and Civilization programs almost always have some sort of overseas program, and even if you only opt to minor in Japanese Language and focus on a different degree, there's still plenty opportunity to keep up the language. I think
さやかさん has a story about someone she knew who spoke Japanese fluently, had never been to Japan but had immersed themselves in the language entirely.
So yeah, unless you're aiming to take exams in Japanese, go over the "all conversation" tracks, do some light reading, gaming and maybe join a fansub group. Oh, and help on these forums. And then when you're at university, see how free you are to choose elective courses with respect to whatever you're doing and then have a Japanese Language component.