Record words and short phrases you think your readers might type into a search engine to find you and your book. Eliminate less important and more generic words and phrases from your list. The more specific the keywords, the more likely the person searching for those specific words will be happy to find your book among the search results. Enter your keywords and phrases to find books like yours on Amazon, and if those terms do not return books that are like yours, try again. Then use the Google Keyword Search tool to help you think of other keywords for your book. Google Keyword Search will show you how popular those keywords are (volume data) and help you find keywords with lower competition. Try to keep the number of repeated keywords to a maximum of three. Work on this task until you have a final list of no more than 10 to 20 words.
Enter the most important keywords and phrases first. Settle on 7-10 keywords and/or phrases and list them in descending importance. Return to Google Keyword Search every few months to evaluate new trends, check popularity and volume, and to improve your keywords list. These same top keywords and phrases should appear in your book descriptions wherever possible.