And to add another point, I think the important thing is not just getting in the mood to write and starting the writing process, but also ensuring that your writing is comfortable and painless. If you get frustrated or annoyed, or if it's not coming together, your motivation can dry up, and that block of time you set aside can wind up being totally unproductive.
For me, for a long time, I worked really hard on getting stuff to sound right. Get that sentence perfect, get that idea conveyed the way I have it set up in my head. Problem is, I got so worked up trying to get the grammar and the structure and the tone perfect that I got frustrated, and very often I'd wind up putting a story away before I'd actually told it. Then I'd rarely if ever pick up the motivation to go back to it. I did this for *years*. I have no less than a hundred unfinished story ideas sitting in folders on my various computers, some scrapped after half a page.
What's really worked for me is simply getting the ideas on the screen. Sitting down, writing, not stressing about errors, not letting myself spend more than 3 seconds figuring out how to word a sentence. What I've found is that by getting into the flow of it, relaxing and letting the story happen, the writing naturally improves as I work. Anything that doesn't come out right, I can fix later, after I've finished and taken a break to do something else. It's often easier and lower stress to fix that sentence or figure out the right word, after, because I've already got the chapter out there, maybe some ideas have come to mind while I've written it, and I'm less frustrated because the time I'm spending editing or revising isn't time I'm taking away from the actual writing.
This is actually part of why I started doing a web serial, because it keeps me moving forward and keeps my focus on continuing the story, rather than continually revising it until I'm sick of it. (This has the unfortunate side effect/self imposed restriction where I won't let myself go back and start editing the first 10 or so chapters of Worm, even though I think my language has improved over the half year I've been writing it, and some wording & stuff really bugs me whenever I go back & reread it.)