My girlfriend is a non-native English speaker doing graduate studies here in Australia. Her English is very good, but it seems there are always subtle mistakes, and unfortunately in formal writing a lot of scrutiny is placed on perfection. My advice would be to have an educated native speaker proof read every article you write. With every correction, try to get an explanation either from them or by doing your own research. In other words try to establish if it's something that can be generalised or if it's an exception that needs to be remembered. Make a list of the common errors you make, and each article try to remember them and avoid those mistakes. Repeat this process. Your English seems to be of a high aptitude, and at that level I think this is the only way to perfect it. It's the way native speakers also learn to write well. We go through this exact process while at school (but less formally, as the list of exceptions is subconsciously absorbed). I think all the conversation in the world won't help a great deal. Conversation is very forgiving, terse etc. There are a lot of native speakers who write poorly. Writing is definitely a much more difficult challenge.

I second Steven's advice, as well as adding: "Read, read, read and then read some more." Get a taste for written English: every writer begins as a reader. :)

based on my experience about writing articles in english, most important are the english grammar and the structure of the paragraphs and you have to use the appropriate words in your article.