1- Write, write, write!
I think to improve your writing you really need to write something in English more or less every day, for example just write a few lines about what you did or about your opinion of something.
2- Keep adding phrases to your vocabulary.
You also need to keep reading and studying vocabulary to gradually make your language more sophisticated. Studying collocations and single words when you're learning vocabulary
5-Take care when you're writing.
Check punctuation spelling and grammar.
Here you are some more tips to help you.
Writing tips de anarosaleda
And please, check these two links which are also very interesting.
And, in addition, you can make use of lots of mobile apps to improve you English on a daily basis.