I have a collection of writing books that I return to whenever I need inspiration. One of my favourites is Becoming a Writer by Dorothea Brande, published in 1934.
I've got a lot out of this book over the years, but the advice that has most helped me is Brande's suggestion of early morning writing. She says the purpose of this is to derive 'the full benefit of the richness of the unconscious'. I look at it as getting up before the policeman who censors your work is on duty.
The idea is to wake up and, without talking, reading or drinking coffee, begin to write, allowing yourself to just fall onto the page. Then stand back and watch what you produce.
Throughout your writing life, whenever you are in danger of the spiritual drought that comes to the most facile writer from time to time, put the pencil and paper on the bedside table, and wake and write in the morning.Another of her suggestions is to make an appointment with yourself to write at a given time of day and then, come what may, write for fifteen minutes at that time. Each day make such an appointment but at a different time, again for fifteen minutes.
There is a deep inner resistance to writing...The unconscious does not like these rules and regulations until it is well broken in to them; it is incorrigibly lazy...and given to finding the easiest way of satisfying itself. It prefers to find its own occasions and to emerge as it likes...you must learn to disregard every loophole the wily unconscious points out to you. If you consistently, doggedly, refuse to be beguiled, you will have your reward. The unconscious will suddenly give in charmingly, and begin to write gracefully and well.
Brande has other advice, for example:
Decide for yourself which friends and social activites are good for your writing, and which are not
Discover the reading matter that stimulates your writing, eg poetry, fiction, non fiction
Because old habits are strong and jealous, teach the unconscious to flow into the channel of writing.
All of this is helpful, but the early morning writing is vital I think. It really is a net to catch the unconscious.