This is one of those things that I only found out a few books in to my writing career because some helpful person told me. So, I’m passing the info on in case it’s helpful to anyone else who hasn’t worked out how to do this yet.
You know that section in a book listing where you can see Editorial Reviews? If a book has them, they appear below the ‘Customers Who Bought This Also Bought…’ section, and above the Product Description. Here the ones I added for The Law of Attraction.

NB: You can add editorial reviews for your books whether you’re self-published or with a publisher. Some publishers may already do this for you, so it’s probably best to check with your publisher before you add your own review quotes. But you can access this feature in Author Central without a publisher account, so it’s available to all authors.
How to add review quotes
Log in to your account at Author Central. If you’re a published author and you don’t already have an account at Author Central, make one (and make one for all the Amazon stores that have one: US, UK, Germany, France and Japan). It allows you to manage your Author page at Amazon, and is also useful for a variety of other things. Once you’ve created your Author Central account, you need to claim your books as yours (this is easy to do), then you can edit/change things as needed.
Once you’re logged in, click on the ‘books’ tab at the top. Find the book you want to add reviews for and you can add them in the Reviews section on the Editorial Reviews tab. Choose a few good quotes that make your book sound awesome, and make sure you remember to credit the blog/reviewer. Once you’ve added them, preview to make sure the formatting hasn’t gone screwy (Amazon can be a little weird). Then save the changes. The reviews should appear on your product page in a day or so.
I hope this is useful to someone. I was thinking of doing a few more posts like this with useful tips/advice/info for authors, whether self-pubbed or not. If you have any questions, or suggestions for things you’d like to see in an ‘Author Tips’ blog series, please let me know in a comment and I’ll do my best to help 🙂