I made a post a while back on how not to talk to book reviewers. Now here's one on how you should. (I've written about this before, but it never hurts to repeat.)
1. Know who the reviewer is. If you don't know who you're asking for a review, then why are you writing to them?
2. Address the person by name. At least have the good manners to find out who you're writing to. (And if you're going to do this by mail merge, at least make sure it's not blatantly obvious.)
3. State upfront why you're writing. For example: "I'm a mystery author with a new book coming out and I'm hoping for a review." (You can say it more elegantly than that.)
4. Give the pertinent details: Author's name, book title, publisher, format (hardback, paperback, etc.), publication date.
5. Include a short synopsis. No more than a couple sentences.
6. Briefly include any interesting hook that might appeal to that reviewer: you're a local author, it's a debut novel, you're an FBI agent, you just won the Edgar. (If you don't have a cool hook, nevermind. You don't have to have one.)
Things you shouldn't include:
2. Press release.
3. Excerpts from other reviews.
4. Long summaries.
6. Claims that this is the best book I'll ever read (and other assorted puffery).
Remember: your goal is to make it as easy as possible for the reviewer to say yes. Reviewers always have many more books than they can possibly read, much less review, and they also tend to be cranky SOBs. Don't give them a reason to say no before they've even considered the book.