The best way to improve any type of writing is workshopping. Find other people writing similar material, and offer to give them feedback on pieces they are working on if they will do the same.
There are plenty of established online workshops for fiction, but with a bit of searching you can find people willing to do this for any type of writing, non-fiction, essays, reporting, technical writing, etc.
All the practice and theory or technique books in the world won't help you much if you don't have an audience to let you know what's working and what isn't.