So here goes (geared to novel writing as this is what I do):
- Read a great deal, but only for pleasure. Reading books you don't like will only make you grumpy.
- Get it right: correct spelling, grammar and punctuation matter. Its and it's, your and you're used interchangeably are unforgivable. If uncertain, Google the rules and learn them.
- Cut into the scene when it becomes interesting, cut out when you have achieved what you set out to.
- If you get bored when reading your own writing, this is a Bad Sign. And you will need to read it a lot as you tweak, polish and hone.
- Brood about your characters - I do this in the bath, driving or walking. Even minor characters like the hero's mother or the dog have their own agenda.
- Remember the villain thinks he's the hero.
- Readers are smart - trust the reader. Never repeat yourself, and let readers do some of the work. They like that.
- Pay attention to criticism in case you can learn from it, but don't feel obliged to take advice. It's your novel.
- And most importantly: Never, ever bore the reader.