Most people would tell you to take technical writing classes, like argumentative writing. It helps, but I learned most of my writing techniques through literature classes that required a couple of papers and where tests are in essay form. I've taken American Lit, British Lit. You basically analyze different stories, which strengthens comprehension skills and opens you up to different styles of writing. Right now I'm taking History of the English Language, which helps a lot because you learn how writing evolved and how vocabulary/grammar changed throughout the years, which does wonders to change how you view sentences and word structure. I took a poetry class last year; it was hell, cause I'm not particularly creative and they made me read all my poems aloud and then I had to sit there while every classmate, one by one, critiqued it. It does help to broaden the way you communicate, though, and it helps you see all the different interpretations that can be found in language.