"If you're trying to persuade people to do something, or buy something, it seems to me you should use their language, the language they use every day, the language in which they think. We try to write in the vernacular." - David Ogilvy
Easier said than done, but highly important.
Why is it so hard for so many of us to write the way we speak?
Because we've had it beaten out of us repeatedly, from kindergarten, through high school, college and sometimes grad school.
We've been criticized so many times for not following an artificially rigid writing format that we've internalized the critic.
But... that critic has been working overtime. The critic needs to go on vacation when you're writing copy.