I was commissioned to write a non fiction book years ago by a major publisher. It was really hard work, and I had to find ways to motivate myself - e.g. get 1,000 words done by this evening and you can go for a pint, or you can treat yourself to a scone and tea once the chapter's done.
During the course of the writing, I ended up with several editors, who grew increasingly useless, so I got some people with different skillsets to look over my material and I got some really useful feedback.
Funnily enough, one of the best lessons I learnt about writing was something I learnt from life drawing class: you may have drawn the hand amazingly well, but if the rest of the body isn't good, bin the lot and start again (i.e. you may have a couple of quite brilliant paragraphs that you're proud of, but if they don't fit in with the flow or the plot, dump them!).