The first step to saving money on college textbooks is to ask around in all your classes and try to find students that are planning on taking any of the classes next semester that you are in this semester.
Once you have found students, offer to sell them your textbooks from your current class. Sell them your book back for a little less than you payed, because if you try to sell it back to the bookstore you will most likely get less than half of the original price that you paid.
Next try to find people that are taking the classes this semester that YOU need next semester. Offer to buy their book back for a little less than what they paid. This is a mutual concept. Let them know that they will get more money for their book by selling it to you than if they sell it to the book store.
Many online book stores sell college textbooks and they are much cheaper than buying from a college bookstore! Be sure to order the correct edition because your teacher may want you to have a specific edition.