The short version is “To be a writer, you must write”, which I quote here from James D. MacDonald’s excellent ‘Uncle Jim’ posts on AbsoluteWrite.
Also: “Rejection is nature’s way of telling you to write a better book.”
For those of you starting out, or struggling along in the middle of something, it’s worth reading his posts. They are plainly-written and bluntly simple, but very effective. Stuff I wish I’d had access to – and listened to – when I was just starting to write.