I've been looking into how to improve my writing as well. One of the things that I've learned is that good writing is the result of good sentences. Sentences, not words, are the basic building block. If you focus on writing better sentences you will become a better writer.
I know this doesn't help with how to write better sentences but just having this direction has helped me focus. Hopefully it helps you!
I would agree with this. The only way to become proficient in something is to do it. I've been typing into rectangular <textarea>s since 2002—plenty of time to grow comfortable with the vagaries of textual communication.
How do you become better at communicating your internal ideas externally... without the aid of non-verbal communication? By doing it! Jump into discussions, type, type, type. Read what others are writing, see which communicators you find powerful, observe which ones fall flat in their attempts. Try to imitate the more power communicators. Are they using a certain style of writing that you could leverage?
Learning more certainly helps—"oh, that's an em-dash? What's its function? How can I use it to more accurately translate my thoughts into sentences others read?" Then lock-in that new knowledge nugget by practicing. Work it into your next post/comment/message to a friend.
My key takeaway would be: Just do it. Write like a fiend!
I know this doesn't help with how to write better sentences but just having this direction has helped me focus. Hopefully it helps you!