Excellent observations by Giacomo Giammatteo in his blog post: Watchdog: Is Kirkus Selling Dreams – or Do They Deliver? I got a Kirkus Review for my latest book—Dare I Call It Murder?: A Memoir of Violent Loss—and I believe (one never really knows) it helped get the book into libraries and book stores, so I figure it paid for itself, at the very least. I also see it as icing on the cake in terms of reader purchase decisions. But I also know authors who did not get very good Kirkus reviews, and some who chose not to have their reviews published. I see it as a calculated gamble, as Giacomo says, and I believe I did it with eyes open.
The best sales tool is word of mouth, and that has held true for me—readers recommending my book to others, either through their own reviews (Amazon, Goodreads, etc.) or posting on Facebook and Twitter. I would never recommend those expensive review packages or advertising programs, especially from third-parties, such as ASI, Lulu, or any other publishing service. My only advertising has been the $60 insertion in the Ingram catalog (via Lightning Source), and I figure that was worth it.
I am fortunate in that my book sold well coming out of the gate, and Amazon took it from there, fueling the sales. I think the best thing an author can do, besides having a good product, is starting an online/social media marketing campaign well in advance of publication so the book catapults off the launching pad. If that is achieved, the Amazon Machine will pour more fuel on the fire.