For me, as someone yet to finish a full first draft of a manuscript, the best advice so far has been that Chuck Wendig blog about self doubt. I get so hung up on looking at some past writing and thinking this. is. AWFUL. that I end up being too depressed to go back to it for days. It was depressing that all these stupid, useless words kept getting in the way of what looked so good in my head.
It really helped to know that most authors don't write mana from heaven prose straight off the bat. Now instead of looking at some of my writing and thinking "this is turgid crap, give up." I can think, "well, I may not be a great writer, but I know great writing when I see it, so I'll just keep polishing till this looks a bit more like it..."