Hi there! Good question! 

There are so many things you can do.. I agree with the others that reading helps but I also think its really important to PRACTICE writing as much as you can - emails, messages, blogs and each time you write try to focus on ONE thing and get that perfect. For example: think about punctuation; then think about spelling; then think about verb+ noun agreement; then maybe check each sentence to make sure you have the subject + verb + relative clause (getting more complex) or that you have added some adjectives and maybe adverbs to make them stronger or weaker and so on. 

Maybe you could make a 'checklist' and go through it each time you write something. Lots of teachers provide an editing / correction service - I'd be happy to help you!  Good luck 

Helen 

I can think of nothing but "practice." But you need a reason to practice.

From about 2003 to 2008 or so I was a fairly active Wikipedia editor. I found that writing Wikipedia articles--and adding sentences and paragraphs to existing articles--really made me concentrate. It made me work on thinking about how to say things and how to be clear. The reason is simply that I knew (or believed) that someone would be reading what I wrote.

So, figure out a way to write articles that someone else will be reading. You could start a blog. If you belong to a club, you could contribute articles to their newsletter or website.

Ugh - my written Spanish is SO bad.  I am working on it now and I am like level B2/C1 but it's so bad.  I have always studied Spanish on my own, never had a teacher, but I am wondering if I need one to get any further. on my own

My advice to you is to use Italki's Notebook section to write blogs that are 3 - 4 paragraphs long, and you can have them corrected by natives.  My writing has improved a lot this way.  

Good luck to you!