Register for your classes early. The sooner you know your courses, the sooner you can find out the required texts for them and begin your research. If the book list for your classes has not been made publicly available yet, email your professors and see if they will confirm the title and edition of the text for their classes. The more headway you have to find affordable options, the better.
Browse the campus bookstore with your required textbook list handy before you buy anything. Look to see whether there are used copies of your textbooks for sale and how much they cost. Compare the cost of the used books to the new ones. Make note of the ISBN (International Standard Book Number) located next to the bar code on the back cover of most books. You can look up books on the Internet using the ISBN to make for easier cost comparison than tracking down each book by title, author, edition or year.
Ask your professors if the newest edition of the textbook is absolutely necessary. Often times an editor will release a new edition of a book with only a few minor changes. If your professors tell you that a previous edition will be fine, look into pricing for those or ask students who have taken that same course in the past if they still have their books and would let you purchase them.
Search for books on the Internet. If you have ample time before classes begin to allow for shipping and handling time, you can probably order your books online for less than they will cost at the campus bookstore. You can order new or used editions of textbooks online. Always take into consideration the cost of shipping for an online bookseller, as the money you are saving by buying books online might be offset by the shipping costs. Use the list of ISBNs you made at the campus bookstore to locate books online more easily.
Purchase books at the campus bookstore. If classes start tomorrow (or yesterday), and you need your books in a hurry, the campus bookstore is probably the safest place to go. They will stock every book on any course's reading list, and you can usually buy them immediately.
Buy books from your peers. School-sponsored book buybacks are usually only for a short window of time, and students busy with final exams and moving at the end of the school year often forget to sell back their books in time. Post a flier on campus or ask your friends if they know anyone who took any of your classes the previous semester or year and offer to buy their old textbooks.