Once your book is out, you need to spread the word as far and high as you can. You can reach your network, but unless the book goes viral through word of mouth, that network will run dry after a few hundred (or thousand, if you're really popular) mentions. And of those mentions, only a very few people will buy the book at first. It takes time, patience and perserverence to see the book to even moderate success.
One way to boost sales, and to at least remind people that your book exists, is to get reviews. Space them out. This way, when you get a new review, you can post it for your network to read, and they won't be spammed with 30 reviews in a day, then hear nothing about it again. You'll effectively remind your friends without bothering them. Amazon and Goodreads reviews can make or break a new author, so make sure to keep checking for them.
Search out review blogs. When someone who doesn't know you reviews the book, they open you up to their network, adding it to your own. This is so important.
There is a certain protocol when asking another person to take time out of their life to read your book and write about it. Remember, they're doing you a favor, whether they like the book or not.
Here's how to get reviews from book bloggers, and what to do when you get them.
1) Finding the blogs: Use twitter, and book hops to find book bloggers. They're everywhere, and they're popular. You could even do a google search to find them. Finding book blogs isn't the problem. Finding book blogs that will be interested in your particular book is a bit more challenging.
2) Once you have a list of blogs you think on first glance might give your piece a favorable review, read their review policy. I cannot stress this enough. You'll easily see if they're not taking your genre at the moment, or if they're only accepting free submissions from publishers. You'll see what format they prefer. They may have specific questions they want you to answer in your email. It will be clear if you don't read their policy. They took time writing it, they expect you to read it.
When you do, look for the pertinent information. Are they interested in your genre? Even if they accept it, if it is not one of their favored types of literature, you may want to look elsewhere. Why waste time for both of you? Do they have a long waiting list? How many followers do they have? These two questions go together because a following is not necessarily a good indicator of how popular a book blogger is. You'll want good, thorough reviews from people who have actually taken the time to read your work, not a mill that pumps out 20 reviews a day. Don't fret about a wait time, if there is one. You've got the rest of your life to sell this book.
If you contain all the qualities they ask for in their policy, send them an email or fill out the form they give you before you send the book. Give them a chance to accept it or say no before foisting it on them. It's only polite. Plus, you'll get the people who really might like the book replying to you because they've taken an interest.
When you send that email, don't form-feed it. Give each reviewer a personal note that shows you're familiar with their blog and the hard work they do. It will help you bridge a relationship early on. It will help both you and the reviewer form personalities beyond the impersonal messaging. Always remember, you're essentially asking a favor of a stranger.
3) Check to see where the review will be posted. Some reviewers only post to their blogs. Others will give you a Goodreads and Amazon review as well.