Here is a question from the interview I did with Patti McCoy Jacob that was cut for reasons of space. I thought it might be interesting to some.
PMJ: Any advice for someone interested in reviewing books but unsure how to begin?
DJM: Read, read, read. Read everything you can get your hands on, including reviews. Find critics whose work speaks to you and study their reviews. Figure out what they're doing and how they're doing it. Then experiment with expressing your own thoughts about a book; what was good, what was not-so-good. You not only have to be able to form interesting insights about the book, you have to be able to express them in an interesting way. It's something few people seem to be able to do well.
I would also advise people that the only reason to write reviews is because you love books and have thoughts about them you want to share. Don't do it because you think there's money in it. There isn't.