Hi,
For me, whatever the language I learn, the bast way to learning to write is reading, reading and after writing for tournament, but is fundamental you reading, can be books, news paper, well what you do like...
I am a Brazilian native speaker, and this can be use for your one language too because, in Portuguese (Brazil) for example, a lot of peoples don't know how to write very well, same in Japanese ( I am learning too) with kanjis, and this can e fix if you reading...
And for training write, one way I like to use is help, can be here or in community like Live Mocha, and help native English speakers who want to learning your language, same time you help somebody you write in English and also can ask for they help you too.
Well, is it!
I still learning English so forget my mistakes to write...
See you!