The day before, decide when you will write and what you will say. Pick a specific time and a definite message. Decide also, in rough terms, how you want to say it and who you want to say it to. Finally, decide how much you will write and for how long. Make the decision at the end of the day and carry it out at the beginning of the next.
For scholars, I recommend deciding to write well-defined paragraphs, each of which says one thing you know to your peers. Each statement of your knowledge, each knowledge claim, can be expressed in a simple declarative sentence, which we call the "key sentence". A paragraph will support or elaborate it in at least six sentences and at most two-hundred words. It will take you exactly 13, 18 or 27 minutes, as you choose. Working no more than three hours, and no less than thirteen minutes, you can write between one and twelve paragraphs per day. That can be as much as 2400 words.
For bloggers, I recommend organizing the work around individual posts of between 200 and 500 words, working no less than 13 and no more than 27 minutes, followed by a short break, after which you get on with your day. There are many ways to plan your posting schedule. You can write every day, but post every week, giving you five sessions on each post. Or you can simply get into the habit of writing and posting every day. This depends both on what sort of writing you want to get good at, and what sort of blog you want to maintain. Keep the amount of time fixed. Keep your message focused every time you write. Think of your readers. With time, you will train a workable style.
My basic approach is to separate the writing process from the creative process that produces its content. Choose something to write about that you know well enough to let writing, not knowing, be the central problem during your writing moment. It takes a little discipline, and if you want a coach I am here to help. But at the end of the day the important thing is that you will write tomorrow. Choose something you knew was true already last week. Put a weekend between your genius and your prose.
In any case, always remember your readers, each in their own reading moment. This is a moment that you have designed for them. You are in complete control of what happens to them from word to word, sentence to sentence, paragraph to paragraph. For every minute the reader spends reading, you have spent many more, writing what they read. This overwhelming asymmetry of effort and control is the whole purpose of writing. As a writer, you are in a position of absolute strength. Don't forget this as you write.