Languages are like plants. If you don't take care of them correctly, they're going to be destroyed. I mean, you're going to forget it completely. It's not enough just to know how to read/speak. If you want to improve your writing skills, I strongly recommend you to write about anything you like: food, fashion, cars, music, movies, ... here on italki in "notebooks" so we can correct you. Also, you can start blogging or you can participate in some forums. If you need English classes, I'll be so glad to help you. Good luck with your studies.

I know this might sound a little bit cliché, but it's the same way you to obtain fluency in a language you do it more. If you need to get better at writing you write more, but also have someone review your writing. Same way with a foreign language, you have someone review the way you talk and write in a different language and continue to practice. Good Luck