I've been involved in a bitter dispute with my Local Government Council for about three years on and off and, in that time, in various letters of complaints/emails, I've probably written about 100,000 words. Looking back at some of my earlier complaining emails/documents, I can see I was overly verbose and overly ranting, but I can see a pattern of development, a style, a punchier voice that has developed over time, making for more effective writing that I can use in my fiction. So if you ever find something that has made you angry, try to use it as way of improving your writing. I always have something to say on the matters of my dispute. If it really makes you angry, you'll never suffer from writer's block.
I've honed my verbal skills defending evidence based conclusions by posting on the JREF Forum. It's also helped me refine my knowledge on numerous issues when someone challenges your conclusions. Either you find out you were right or you learn something new.
Another way to improve writing is to keep a diary. I write an average of 3,000 to 3,500 words' worth of diary entries every day, seven days a week, and lately there are many days when diary entries are all I write. Hey, it's still writing, so it's still good practice. It teaches you both narrative and lyricism.
I seem to recall a Bentley Little book or Jack Ketchum? I'm not too sure it was a real good book ( so good I forgot the title - lol ) but it was about a man's life changing because of his sending scathing letters to companies. He kept getting all these freebies and benefits. It made for a rather cool book.
Doing film or book reviews on IMDB or goodreads for a film or book you really hate can also be helpful. I'd say anything where the characters, events and plots already exist and you can hone your skills trying to find the most readable, humorous, enlightened or enlightened way possible to get what you want across. I've decided to let go of my complaint and carry the bitterness in my heart for the rest of my life, and just see it as something that ultimately helped me get back into writing.
Every time I'd go through a rough patch in my life, I'd write down all my feelings. I've kept all these random accounts. I love seeing how my writing has improved through the years, despite the fact that the content is entirely stream of consciousness.
Just putting my bad customer experiences into a blog helped me stay sane while also being able to laugh at a situation. So it helped me cope with my job annnd retain a sense of purpose when it came to writing. Win for all!