I just found a great website for saving money on textbooks: BIGWORDS.com.
Find out which textbooks you need for your classes on you school's website (bookstore) and compare to the prices that this search engine finds. It basically searches the Internet for the best deals on the textbook you are looking for. Make sure that the shipping doesn't make it more expensive than buying it at your local bookstore, and remember to factor in your student discount that you would get locally.
Another option - if it is a book that you will only need for a few weeks of your class, borrow the book from someone who has already taken the class or check it out at the library. Don't forget the public library; they don't often carry textbooks, but if you are looking for another type of book they offer better checkout terms. Often professors put their required textbooks on reserve at the library, but they are so overused that it's hard to get your hands on them when you need them.