1. Post Regularly
If you want to keep an audience, keep posting.
2. Write About What You Know
Increase your comfort level and expand your boundaries slowly. Start with what you're most comfortable writing about, and then gradually work up to older stuff or newer stuff, whatever you want your subject to be.
3. Write About What You Own
Start with books you already have, whether you've already read them or you're reading them specifically to review them. The more you review, the more publishers will be eager to offer you copies for review.
4. Request Review Copies
After you've got some material already posted, shop through the online catalogs for publishing houses like Penguin and Random House. Get some ideas, and email the appropriate imprint's publicity department. Including links to some of your best or favorite reviews already posted on your blog is often helpful.
5. Know When To Say No
Sometimes I think the only thing I love more than books is free books. But you've gotta know when to say no. Once you get in with certain publicists, they're going to send you a blast for each of their new books. Say yes, and say yes often, but keep in mind that they want to get the books out there for marketing purposes - they want reviews posted around the time that the book is being published. If that's not a deadline you think you can meet, do both you and them a favor, and say no.
There are a lot of book blogger memes out there...multiply that by about a thousand and that's how many book bloggers there are competing for the same audience as you are. Find the memes that suit you (for me, it's Monday Mailbox and Top Ten Tuesday), and keep them up from week to week. It's a great way to get people to read your blog and to stick with reading your blog. And it'll keep your posting regular. Don't be afraid to be social.
7. Keep Track of the Books You've Reviewed
Both LibraryThing and GoodReads can be very very helpful AND give you more visibility.
8. Everyone's a Critic
Don't be afraid of your own opinions. You have them. Express them. The book publishing industry is a criticism-driven one, just like every other kind of art. Your opinions will not always be popular, but that's kind of the point. But just as you want your opinions to be respected, be sure to respect others' as well.
9. Accept Recommendations
But do not feel obligated to stick to them. Everyone and their mother is going to recommend the next best book for you to read, but that doesn't mean you have to read it. Just as with publicists and their review copies, it's okay to say no.
10. Social Media is Your Friend
I can't explain it, and I don't know how a lot of it works, but using things like Twitter and Facebook and NetworkedBlogs and some other things that, again, I don't know how they work, these things can be very helpful to increasing your traffic and expanding your audience.