Review the rules for that website or publication. In some cases, you may have submitted the wrong material. Some places require two copies of a book, while others may accept unfinished galleys and others will not. Some places may be aimed at a market your book does not fit. If the site focuses on fiction, it may not be a good fit for a history book.
Address your reviewer by name when possible. Let the reviewer know you have read his material and like what you see.
Explain the title and premise of the book in a precise way. Some writers are so excited about their own work they try to sum up the whole story in the release. However, the reviewer has perhaps a minute or two to read this note and the other 100 notes she will get that day. There is a good chance the reviewer won't even bother to read a long request. A good writer should be able to sum up the book in less than two paragraphs.
Mention any prior honors and positive reviews. Provide links to those reviews if possible. Thank the reviewer in advance for any consideration. Include a link to a photo of yourself or the book cover in the email, or a glossy photo in a mailing. This will save the reviewer some time should she want to illustrate the article. It will also put a face on the release, which can be helpful.