Making Your Own Databases Vs. Using Pre-Made Databases
There are some decent pre-made Japanese databases for Mnemosyne, and I am working on some more myself. But, although these can be helpful to start, they're not going to help you most in the long run.
The most effective way to learn is to make your own cards. The reason for this is very simple: you know best what you don't know and what you want to learn. Mnemosyne can help you not forget, but it can't help you choose. And although pre-made collections can be useful if they contain exactly what you want to learn (all the prefectures of Japan, for example), rarely is this the case.
Also, even in cases when the collection does include everything you want (all the JLPT2 words, for example), it lacks order and priority. Some of the words on the JLPT are going to be easier for you to remember than others, and these will be different than everybody else's. So, if you're making the cards yourself, you can easily do this. But if you're using someone else's cards, it's a lot harder.
It seems like it takes more time to do it yourself, but paradoxically, that's the fastest way. If you feel like you're adding too many cards and it's taking too much time, ask yourself what you're memorizing. Is it really what you need to know? Do you need all of it? The simplest way is usually the best-- and the most likely to be remembered.