Getting a book reviewed is vital for your marketing plan.  Book reviews, regardless of the rating, have a variety of important functions.

1. Book reviews raise awareness about your book.  Readers can't buy what they don't know about.  Readers who read book reviews do so because they are interested in buying books. 

2. Book reviews provide information about your book.  While banners and ads can make someone aware of your book, a review provides information that readers can use to make a decision.  Particularly on the internet, people increasingly tune out to banners and other forms of advertising.  But reviews are considered news, and as such are read by those interested in books.  Even if a reviewer doesn't like a book, the information in the review can lead a reader to look for more information.  Or, the things the reviewer did not like may be things that do interest the review reader.

3.  Book reviews provide immediate access to your book.  Most book review sites raise money through affiliate programs.  This means most will at a minimum include an Amazon or B&N affiliate link along with your review.  Many sites include links to the publisher or author's homepage to provide readers with more information.

4. Book reviews provide credibility for your book.  Many libraries and bookstores expect to see excerpts of book reviews with media kits.  This helps them determine which books to stock.  The old saying "a bad review is better than no review" definately applies.  A well-written, but bad, review will still point out good things about your book.  And these may be enough to get a librarian or manager to believe your book still has some general value.

5.  Book reviews provide feedback.  While a single review is only one person's opinion, if the same things come up in multiple reviews, that should be a sign to an author of a problem. 
Things to Remember:

When you query, be professional.  Your query will color the reviewer's impression of the quality of your work.

Getting reviews is not just about publicity.  It's also about building relationships.  Leave a good impression on a reviewer and they will remember you, which means future interactions with the person will be easier.  Leave a bad impression, and not only have you destroyed any future relationship with that reviewer, but possibly relationships with others who the reviewer talks to.

Suck it up if you get a bad review.  Reviewers aren't out to get you.  They are merely presenting their opinion on your book.  Unless the review is malicious (i.e. calls you names, is sexist/racist/etc), let it go and move on.
Pre-Request Actions:
Make sure you have a professional face to put forth.  Is your website ready?  Do you have clear contact information?  Think about the image you want to put out to the world as an author and publisher.  Do you want to be seen as Janedoepublishing@domain.com, or as sexkitten666gyrrrll@domain.com?

Research potential reviewers.  Don't just give copies to anyone who says they will review it.   There are thousands of people looking to get free books by offering to write book reviews.   Some really think they do a good job.  Most are only going to be seen by other writers looking for reviews or the reviewers friends.  Remember, you have a finite budget, so look into the potential reviewers to see where you will get the most bang for your book.

Read other reviews on the site.  Get a feel for how the reviewers write their reviews and what they tend to look for.  In some cases, you may be able to request specific reviewers.  Knowing which reviewers really enjoy cheesy chick lit can be important when requesting a review of your cheesy chick lit novel.

Making the Request:
Follow the guidelines.  Most reviewers are not professional reviewers.  Like most writers, they have day jobs, families, and lives.  Guidelines exist to make their lives easier.  Anything that will deliberately make it hard for them if going to count against you.

Keep it short and to the point.  Give the reviewer the basic information they need, such as a summary, page count, genre, and ISBN.  Provide a link to your website for more information.  Don't try to recreate your website in the query.

Avoid chatspeak, and check for spelling and grammar errors.  Nothing will turn off a reviewer quicker than opening up a query to read "id lke 2 have U review my book plz."

Ask for the review.  Sometimes reviewers aren't sure if they are reading a spam book promo or a book review request.  Make sure the intent of your communication is clear.  For example, end your query with "If you have a reviewer that would be interested in reviewing this title for your site, please let me know and I will be happy to send out a comp copy."

Sending the Book:
Unless you are instructed otherwise, do not deface the book.    This is the deal.  You are asking a reviewer to spend a week or so of her life reading and then writing a review of your book.  You are asking that reviewer to give you free publicity on her website.  That book becomes the reviewer's property.  If she wants to donate it to her local library, sell it on ebay, or give it away as a Christmas present, that is her business.  Slapping a big sticker on it that says REVIEW COPY NOT FOR RESALE says to the reviewer "I don't trust you."  And that is not a way to build a relationship.  By the same token, a personal note on the inside cover thanking the reviewer (by name) says "I appreciate what you are doing for me."  Which statement will put the reviewer in a postive frame of mind?

Include a cover letter that restates any previous communication.  People get busy.  The mail runs late.  Things happen.  Your cover letter reminds the reviewer of your previous conversation and what is expected by both parties. 

In the U.S., I suggest sending the book priority mail, not media mail.  It costs a couple dollars more, but it also gives you tracking information. 

After Sending the Book:
Follow up a week later to make sure the book was received.  If you sent it priority mail, you have the tracking number. 

Do not send multiple queries to the reviewer.  Most reviewers will provide you with a timeframe.  If that deadline passes, wait a few days before following up. 

After You Get The Review:
Thank the reviewer for his time.  If the review was bad, thank the reviewer for his time anyway.  Don't argue, threaten, or whine about it.  Many review sites no longer review POD or self-published titles because too many authors have behaved in an abusive manner.  Unfortunately, that includes us.  Due to increased hostility and threats from authors who were displeased with poor reviews, Bards and Sages has stopped offering its review service.  We still review books that we buy, but will no longer review books on request.  If you learn nothing else from this site, know that your behavior impacts not just you, but the entire independent community.  You are representing ALL self-publishers when you reach out to a reviewer.
Step by Step Guide to Obtaining Reviews: