I don't write linearly so, if I get stuck at a certain point, I skip it and write the thing that I actually do know how to write. Hopefully, by the time I finish that, I'll have figured out the thing I was stuck on. Another good thing is to give myself permission to write a terrible first draft. It's much easier to then go back and attack the book once there's something on the page instead of when there's nothing. And much more liberating!