If you're already taking Japanese in school, you're on the right track. If your classroom isn't already using the Genki textbooks, I suggest picking those up, as I've found them to be the absolute best books for learning the language.
To boost your listening comprehension, I suggest downloading raw anime without subtitles. I worked on fansubbing Fullmetal Alchemist for a few of my friends back when that was airing, and saw a huge increase in my vocabulary and grammar after researching any new words and forms that I came across. Of course, I was already decent at the language by then, so I suggest something easier like anime for younger kids. Doraemon would be a good start to gauge your level.
For reading practice, first you'll need to learn katakana, and you probably already have the resources. It took me about three days with lots of practice to have it down, so if you study hard, that should come easy. Kanji is a different beast entirely, and a simple routine would be to learn one a day. Again, the Genki books will help here. Pick up as much raw Japanese manga as you can to boost your reading speed. Older video games work well, too - Final Fantasy IV and Mother 2, for example, are both entirely in kana.
If you're looking at picking up some DS software, I recommend "Kanji Sono Mama" above all others for kanji practice. I use it mostly for recognizing kanji I don't know, because it allows you to write in the kanji (or combination of several kanji) that you want the definition for, then displays both the kana reading and English translation. Super-handy!
Oh, and above all else, study regularly. There is no shortcut. Study abroad if you can, too! Good luck!
