Print a copy of the books you will need for the upcoming year or semester at least a month in advance. This will help you avoid the rush of students buying up the more reasonably priced books before class begins. If your list is not available, try to get in contact with the professor for the class whose book list you do not have yet. Professors are there to help you, and most of them will be glad to tell you what books you will need.
Check at least three different bookstores including your university bookstore, and write down the prices of all the books you need at all three locations. While university bookstores are usually more expensive, some of the books you need may only be there. Consider using online stores if you have time for shipping, but only buy from reputable sites such as Amazon.com or Half.com. Unless the book is for a lab or must be new for some particular reason, it is best to purchase a used one if it is available.
Rent books rather than buying them. There are sites online that offer textbooks for rent, such as Chegg or Barnes & Noble, although their selection is somewhat limited. Libraries are another potential book rental source -- they may not have the textbook you need for your Algebra class, but they might have the copy of "Hamlet" you need for an English class.
Purchase the books you need from the locations with the best prices. When you get home, put covers on the books to keep their covers protected. Try to keep them in good condition throughout the year, so that you will be able to sell them back if you wish.
Sell your books back at the end of the semester, unless you think you will be able to use them in classes you will take in the future. Although you can sell books back more quickly to local vendors, you can usually get better prices through online auction or even direct sale through advertisement online. Many college bookstores also buy back books at the end of a semester or quarter.