Dear Friend,
If you want to improve your written skills try to practice as much as you can. I mean try to draft letters/mails on diverse of subjects. After that show the same to experts who are sound in English, and request them to correct the same. Afterwards you have to keep in mind their corrections, while writing next time try to avoid the mistakes. Conclusion is Practice makes perfect so try to write you as much as you can.
I wish you all the best.