One of my favorite books on writing is Fast Fiction: Creating Fiction in Five Minutes by Roberta Allen. The book is essentially a workshop in a book on creating flash fiction (what was more commonly referred to as short-short stories in 1997, when this book was published), and also provides guidance on how to take the pieces you create and turn them into even longer works. The early chapters of the book focus primarily on the short-short/flash fiction, what makes them work, and how they compare to traditional, longer stories.
The part of this book that you keep coming back is the section of writing exercises. She gives you hundreds of writing prompts. For each prompt, you get five minutes to write a story (a sample prompt—Write a story about stairs). Since it’s an exercise in spontaneity, it’ll be rough, but you’ll have written something that can later be refined into something better.
I have used these exercises to get me writing, and I’ve reworked the material from many of these five minute pieces into works that have more definite shapes. Multiple works from my 212 series started out as five minute exercises. I have a store of old exercises that haven’t been reworked yet, but this accumulation of raw material will be quite useful in writing future stories.