Harsh, brutal critique from other people (ideally the target audience), prompted by deliberate questions from us about how easy it is to read and understand, and questions from us to test that understanding.
Writing happens entirely inside one's own mind; it can be so hard to see that what we're writing isn't clear, isn't well structured, doesn't guide the reader deftly through understanding and enlightenment. I wonder if it's even possible to properly critique our own explanatory pieces on technical issues, given that we cannot read it without having that knowledge already in our heads, as the target audience will have to.
It can be quite a shock to learn how badly written some of our own work is; as with code reviews, ego has to step aside.
