The New York Times Bucks Blog has a great feature on finding textbooks for less. There’s a great list of comparison sites in there. Don’t forget too the option of e-textbooks at sites like CafeScribe. Having all the text digitized not only makes it lighter and more portable, but you can easily CTRL-F if you’re trying to find a key phrase or concept.
Finding Cheaper Textbooks: 2nd Edition [NYT Bucks Blog]