Check your book list to identify the books you need to buy. You can also contact your professors ahead of time, or on the first day of class, to see what books are required. They may also post some reading snippets online, eliminating the need to buy the book.
Avoid going to on-campus bookstores, which generally charge full price for books.
Compare prices for your books on textbook websites (see Resources). You may find the price is significantly different from one site to another. Used copies of the books are also cheaper.
Ask your professor if you can purchase an international edition. You can also look to see if older versions of the books are significantly different from the newest. Sometimes only minor changes are made, and you can get the older editions for much less.
Search textbook websites for coupons. You might be able to save some money on an order over a certain amount of money. Free shipping is also an option, which can save money, because textbooks can be quite heavy.
Share with a friend. You and a friend can split the cost of a book if you're going to study together and take the same section of the class. Work out a schedule so you both have the book when you need it.
Sell your books back to the bookstore or online to pay for the next semester's books.