Think about the main point of your book. For example, some books have a certain moral or are marketed for a certain group.
Compare your book's title to the focus of your book. Determine if there is an aspect of the book that the title doesn't get across.
Think about keywords that might describe your book. Adding these to the subtitle will help the title to show up on Internet searches.
Write a first draft of what you think a good subtitle might be. For example, if your book is written to help teens handle divorce and is titled "Divorce Drama," the subtitle might be: "How Teens Can Cope With Divorce."
Put the subtitle out of your mind for about a week so you can look at it with fresh eyes. Reread the subtitle to make sure it still "pops" for you.
Have a writing partner look over the title and subtitle. Getting a second opinion can help you see where you might be missing something.