This may sound obvious, but rarely is. Writing is the same way. Before your pen ever strokes the first word, you should know what you've got to say. If not, the only thing you'll be doing is stringing random moments together, and praying it turns out all right. This may produce semi-decent art, but it seldom produces solid writing.
Solid writing is real, and the readers can feel it.
It's real to them because it was to you when you wrote it. It has fire and impetus because when you wrote it, you felt like your heart was bleeding out onto the page. You might have been embarrassed, you might have been ashamed, but it was real, and when you read it, you tingle with goose-flesh. Solid writing is good writing, and good writing makes great writing. Writing is much the same.