I think the following advice from John Berendt is great. Try it for a week and let me know how it works for you.
Keep a diary, but don't just list all the things you did during the day. Pick one incident and write it up as a brief vignette. Give it color, include quotes and dialogue, shape it like a story with a beginning, middle and end—as if it were a short story or an episode in a novel. It's great practice. Do this while figuring out what you want to write a book about. The book may even emerge from within this running diary.
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