DO PREP BEFORE SENDING A REQUEST
Research your target bloggers/book reviewers, first. Read their past reviews, learn what sort of stories they like or dislike. A paranormal romance reviewer may only review vampires, not werewolves or ghosts. Don't waste their time (or yours) by submitting a book that is not to their taste. I also suggest commenting on their postings and following them on Twitter (and RTing any of their tweets that interest you) and/or Facebook (and liking, commenting or sharing any of their posts).
AND DON'T ASK FOR A REVIEW VIA SOCIAL MEDIA
Unless you've developed a close relationship with a blogger/book reviewer on Twitter (you've followed each other, exchanged Direct Messages and now email each other regularly) NEVER try to land a book review via Twitter. I recently had a blogger send me a rather nasty tweet as she mistakenly thought that I wanted a review, when I only wanted a retweet.
Her over-reaction made me think that this is a common problem for bloggers/book reviewers. I've also noticed that few bloggers/book reviewers follow back on Twitter, which is a shame as they are missing out on developing relationships that could increase their website traffic, but I understand that they are being overwhelmed and it is unfortunate.
Know that sending an email or submitting a request via a 'Contact Us' form is, by far, the preferred method of communication (it's the only way that I, personally, request book reviews), as you can not convey all the important information necessary for a blogger/book reviewer to decide if your book is appropriate for their venue, in a tweet.
Here's an example of an email that I sent to a Blogger/Book Reviewer to show you what I mean:
ALLOW A REVIEWER TO SAMPLE BEFORE HAVING TO DOWNLOAD
I like to provide a short sample of my work, so that the blogger/book reviewer can quickly determine if my book is to their taste. I set up a private page on my website for this purpose, but you could alternatively link to your Amazon sample page. I assume that most reviewers want to establish the quality of your work before investing too much of their time—that's the purpose of a sample.
PLAN TO USE LINKS RATHER THAN ATTACHMENTS
Next, I provided links to the PRC and PDF formats of my book. I used a free service at www.YouSendIt.com (now Hightail) which will provide access for downloading these files for up to 7 days. Providing a Smashwords coupon code will work, as well, just be sure to include the Smashwords product page link where they'd enter the coupon code. Since at least 50% of these bloggers/book reviewers can only be contacted via a 'Contact Us' form, be aware you'll need a way to send your book files without using attachments.
After the book links, you should provide some background links to your website, social media platforms, and some recent interviews (blog posts will work, too). Your Amazon product page link should also be included as most bloggers/book reviewers will like to see the number of your current book reviews and ratings (some bloggers will not review books with less than a certain number of reviews and the rating cut-off is usually an average of 4 stars).
PROVIDE INSTRUCTIONS ON HOW TO DOWNLOAD
At the end of my email there are instructions on how to download a PRC file onto a Kindle from the YouSendIt link. If I was using Smashwords coupon code, I'd have an overview on how to redeem the coupon. Make downloading your book as simple and easy for a blogger/book reviewer as possible.
PROMOTE YOUR REVIEWS!
Lastly, if you land a book review be sure to promote it on all your Social Media networks. You should tweet about the link with their Twitter address included (so that your peeps will follow them, too). If there are comments, be sure to respond to them. I also recommend linking the review on the 'News' page of your website, so that all future visitors will know that you were reviewed by this blogger/book reviewer. You also should take the best snippets from the review and quote them on your Amazon product page (using Amazon's Author Central).