This is spectacularly simple advice from Derek Sivers. I immediately used the approach after reading this article for a script I was writing for a screencast and it really helped!
My advice to anyone who writes: Try writing one sentence per line. I’ve been doing it for twenty years, and it improved my writing more than anything else.
New sentence? Hit [Enter]. New line.
Not publishing one sentence per line, no. Write like this for your eyes only. HTML or Markdown combine separate lines into one paragraph.