1. To me, the single biggest mark of the amateur writer is a sense of hurry.
Hurry to finish a manuscript, hurry to edit it, hurry to publish it. It's definitely possible to write a book in a month, leave it unedited, and watch it go off into the world and be declared a masterpiece. It happens every fifty years or so.
For the rest of us, the single greatest ally we have is time. There's no page of prose in existence that its author can't improve after it's been in a drawer for a week. The same is true on the macro level – every time I finish a story or a book, I try to put it away and forget it for as long as I can. When I return, its problems are often so obvious and easy to fix that I'm amazed I ever struggled with them.
Amateur writers are usually desperate to be published, as soon as possible. And I understand that feeling – you just want it to start, your career, your next book, whatever. But I wonder how many self-published novels might have had a chance at getting bought, and finding more readers, if their authors had a bit more patience with them?
2. Every artist was first an amateur.
3. An amateur is an artist who supports himself with outside jobs which enable him to paint. A professional is someone whose wife works to enable him to paint.