I read an autobiography of a writer (Jim Harrison) who begins by saying he does an encerrona by checking into a motel. (He doesn't use the word encerrona, of course.) That is one method. A writer's colony or retreat would be another. I think the spatial aspect is important: you need to get away in some way, or spend time in your office on the weekend when you normally wouldn't. Something that marks the time / space as sanctified and devoted only to your work.
If you cannot generate 1,000 about your project in a morning's work, you probably don't have enough ideas, yet, to call it your next book project. This particular project snuck up on me, tapped me on the shoulder, and said, it's my turn now. It had answers for all my doubts and snowballed so quickly that I was humbled (though I hate when people say they are humbled).
I think now what I need to do is give more talks about it. I am available!