Search using keywords you're considering on Amazon. If you get irrelevant or unsatisfying results, make some changes. When searching, look at the suggestions that appear in the "Search" field drop down
Think like a reader. Imagine how you'd search if you were a customer who didn't know your book existed.
Examples (for fiction)
Setting (Colonial America)
Character types (single dad, veteran)
Character roles (strong female lead)
Plot themes (coming of age, forgiveness)
Story tone (dystopian, feel-good)
Keywords to avoid
Information covered elsewhere in your book's metadata (title, contributors, etc.)
Subjective claims about quality (e.g. "best novel ever")
Information common to most items in the category ("book")
Spelling errors
Amazon program names like as "Kindle Unlimited" or "KDP Select"
Other metadata and keyword tips
Focus your book's description on the book's content
You can change keywords and descriptions as often as you like
On Amazon, use the category menu to brainstorm keyword ideas. For example in the Romance category, you can refine search results in the menu on the left by "Romantic Heroes" such as vikings or doctors. If your book is a viking romance, consider using "Vikings" as a keyword.
What are the best keywords for your book?
You are limited to 50 characters for each of the 7 slots (350 characters total). I recommend this slick tool to make the most of the real-estate KDP provides.
Word Counter & Keyword Density - Note that while there is a 1000 word limit for Amazon descriptions, you can take advantage of far more 'sales page real estate' via AuthorCentral. Read more on AuthorDock.