This is a writing technique that is very popular for screen writers. However it seems to work well for writers of fiction and poetry. I always enjoy doing this writing exercise even if I am not stuck. When I am in the middle of writing something else, doing this exercise before going further helps my writing feel fresh.
First do some arm stretches to get your blood flowing. This is important, because the blood flow actually helps you think better. It might not seem obvious at first, but getting your blood flowing will make your brain work that much better because of the extra blood flow reaching your thought-producing brain cells.
After stretching, write down a title on the top of your page. It should not be very long. Often my titles are very cliché and obvious, like The Dark Island, or The Wishing Well.
Next you should set a timer for three minutes. Then you should write as fast as you can until the timer runs out. Don’t try to make sense, don’t try to match the title, just try to write as fast as you can without even thinking about it. If you are writing as fast as you can you will end up writing about unusual things. Things you would otherwise never write about.
After the timer is up, your writing might be hard to read, it might not make sense in places, but you will have created something new and interesting. You also will have warmed up your writing muscles, and will be prepared to work on more serious writing.
If you enjoy this exercise, I recommend doing it daily, keeping close track of your progress. After each session, record the number of words you can write in a minute. If you do this exercise often enough, your writing will get faster and faster. When I started I could only write about 120 words in three minutes, and after about a month, I could write 165 words in three minutes. Not only that, my writing got more polished, even as it got faster.