1. Ask your friends. Ask them why they liked or disliked it, where it ranked compared to other books, if they would read it again, etc. Once you find a friend or two with similar taste in a specific genre, you can really narrow down which books you should read.
2. Find out what your favorite authors read. I have found great books this way and been introduced to some of my favorite authors. For instance, I probably never would have picked up a book by Dave Barry but one of my favorite authors said he was the funniest writer he had read. I tried reading some and have since picked up almost all of Barry's books. Hint: Search your favorite author's websites or blogs for info on who they read.
3. Always read the worst reviews on Amazon. More often than not, it is the one star reviews on Amazon that help me choose what to read more than the five star reviews. A positive review tends to be general, while a negative review almost always dives into specifics. You almost invariably learn much more about a book this way.
4. Use the best books lists. Use the lists online at Amazon, use the countless other lists online, and use the lists in your favorite bookstores. I love bookstores that have the employees choose their favorite books and write a short paragraph about why they liked it. You can find tons of great books like this. My favorite bookstore for finding new books is BookPeople in Austin, Texas. Tons of reviews, tons of lists, and great recommendations.
5. Use Amazon to its fullest extent. Amazon is an amazing online store, but it is much more than a store. Even if you never use it to order books, use it for recommendations. Rate the books you have read, tell it which books you own, read about the books it recommends to you. A lot of the books that Amazon recommends to you will be out of left field, but with a little bit of sorting you can find some hidden gems using Amazon's ranking system.
Of course there are many ways to find books to read that you will like, but I use the above 5 more than any others these days.