The Power of Little Notebooks
Often times you'll hear or see something and want to add it to your Mnemosyne flashcards but a computer isn't handy. For this purpose, I carry around a small, pocket sized notebook and pen at all times.
In the notebook, I write down anything I say or hear that I want to make as a flashcard. Also, if I find myself trying to say something in Japanese and failing, I write down what I'm attempting to say to look it up later (and make a card).
Any notebook will do; the key is that it needs to be portable and you need to remember to take it with you. This is not as easy as it seems; it probably took me two weeks to get in the habit of consistently remembering it.
Once I started carrying the notebook around, though, I began picking up some really interesting bits of information that I otherwise would have lost. Of course, it hasn't dramatically improved my Japanese, but it's been a nice supplement, and the price is right (about $2).
Of course, it's much easier if you're living in a place with Japanese everywhere, like Japan.