Paul Graham, of Yahoo! Store and Y Combinator fame, with some advice for pen writers:
Something comes over most people when they start writing. They write in a different language than they’d use if they were talking to a friend. The sentence structure and even the words are different. No one uses “pen” as a verb in spoken English. You’d feel like an idiot using “pen” instead of “write” in a conversation with a friend.
You don’t need complex sentences to express complex ideas. When specialists in some abstruse topic talk to one another about ideas in their field, they don’t use sentences any more complex than they do when talking about what to have for lunch. They use different words, certainly. But even those they use no more than necessary. And in my experience, the harder the subject, the more informally experts speak. Partly, I think, because they have less to prove, and partly because the harder the ideas you’re talking about, the less you can afford to let language get in the way.
Informal language is the athletic clothing of ideas.
I’m happy to report that I often write like an imbecile, so I’m already following Graham’s advise, more or less.