There is no science to guide a writer to strengthening his or her imagination.  But here are a few practical ways to hone what the writer has been genetically given as imaginative potential.

Live to experience and discover.

A rich life reliably stimulates imagination.

Learn to live actively, not passively.

Reading is active.  Watching TV is predominantly passive.  Listening to music is passive.  Creating original music by composing and/or playing an instrument is active.   Looking at travel photos of France is passive.  Two weeks of backpacking in the Loire Valley is active.

Learn as much about everything you possibly can.

Disparate ideas and unlike associations seem to sprout new images and ideas.

Examine metaphysical questions.

Who are we and why are we here?  Is there an afterlife?  Why do we suffer?  Who is God?  Is there an ultimate truth?  Why is there no justice?  What is beauty?

Musing on the unanswerable helps with character development and significant story meaning that intertwines plotting.

Know your own strengths and weaknesses.

Determine as truthfully as possible how you fit into a world with billions of other unique, vastly different human beings.  This may require painful self-examination.

Practice imaginative writing.

1. Study the great literary creations of the past, and carefully filter out any useless or harmful dogma of contemporary teaching.

2. Explore daily metaphors . . .  the timing of delivery, acceptability, and the logic and credibility.

3. Learn the use of clear and accurate language in all communication, and expand vocabulary with image provoking words and active verbs.

Learn to structure stories and create characters imaginatively.

Discover the reason for success of stories and characters in all forms of storytelling and all prose genres, and then imaginatively create your own new and immediate ideas for success in literary story fiction.