I'm working on a book and I drafted a number of friends as "alpha readers" to whom I send every chapter when it's done. While I'm not on a strict deadline, the desire to demonstrate to them that I'm "still making progress" has been massively helpful.
I've been writing a book and I do something similar. After every writing session I show my partner what I've written. It gives me instant positive feedback and keeps me from spending too much time rereading/editing old chapters or whatever. I've been able to relax on that a little bit as the project goes on, sometimes I have to spend every other session planning or doing research, but at first it really helped me to get the habit going.
I'm writing fiction so this tip isn't as useful to me, but for non-fiction you could try writing a series of blog posts, and then later stitching those together into your book. The posts (hopefully) give you some quick social likes to give you that dopamine hit.
