Lots of indie authors who like to give advice will tell you not to respond to reviews when you get them, particularly the negative ones. I'm not one of those authors. Responding to reviews isn't just a good idea -- it's your duty.
Thank You Cards
When I was little, I had to write thank you cards for everything. Birthday, Christmas, Easter baskets -- I was told to say thanks for it all. I hated it, and swore I would never deal with that nonsense again once I became adult.
And now, I'm going to tell you to do the same thing I hated to do when I was a kid. I want you to write your reviewers, and say thank you. Because you do have to respond, even to the really ugly reviews, and the response is always the same: thank you.
I'm not suggesting that you leave a comment on every Amazon and Goodreads review you get (though it can't hurt), but I am saying outright that you should email every book blogger who features you on their blog. Most of the time, they'll email you to let you know the review is live (so you can promote it for them). This makes it easy -- just hit reply, say thank you, and move on. Otherwise, you might have to dig around on the site to find the blogger's email information.
Do it every time. You show your appreciation because you're probably going to write another book one day. If you want it reviewed, build a better relationship with book bloggers. Say thanks for the review...even when what you really want to say is thanks for ripping my book to shreds.
You're not always going to want to say thank you, or anything at all, and if you do say something you may be in danger of making a nasty remark. Not all book reviews are good, and it's not always easy to be gracious. That's why you should keep your response simple, and standard, for every review. Don't try to come up with new responses. Keep using the same one. Something along the lines of thanks for reading and reviewing my book does nicely for the good reviews and the bad. Show grace under fire, say thank you, and you'll get lots more readers and reviewers.