Once your book is selling, you'll have a steady stream of potential reviewers. Whenever you receive e-mails from readers with praise for your book or requests for further information, you might conclude your response this way:
Thank you for the kind words about my book. If you ever have a spare moment, it would be a great help if you could post a review of it on Amazon and let other potential readers know why you liked it. It's not necessary to write a lengthy, formal review--a summary of the comments you sent me would be fine. Here's a link to the review form for my book: