I've had the same problem! After school I stopped studying Japanese and slowly I forgot things. The biggest catch is doing your research on the internet then getting carried away at looking at something else on the web.... I do it all the time. ;)
What I ended up doing was going back to my school workbooks and re-doing my exercises (though this lasted a couple of weeks :p), then I found an interesting site called sharedtalk.com where you can text chat and voice chat with people to practice your Japanese.
I also use a little flashcard program for the computer called Anki (http://ichi2.net/anki/
) which I find extremely helpful in re-learning all my words.
For listening practice you could try going to the Japan news sites and downloading the video feeds they have on there. You can also get free podcast lessons from japanesepod101.com (listening is free, the grammar files and other extra things are part of a subscription).
For grammar do a web search for Japanese grammar, or just type in the grammar point you want to check and you should find a multitude of websites with heaps and heaps of help with grammar.
Best of luck!!