Gather information on the book that you will need for your coursework by researching the book's title through the campus bookstore's website. If possible, obtain the ten digit ISBN (International Standard Book Number). This will assure that you will not get an older edition with the same title.
Enter the ISBN into an online bookseller, such as Amazon.com, and look for the lowest price listed. Since many of the used textbooks sold on Amazon are being sold by students themselves, the best prices come from a broader search engine, such as shopping.google.com. This search engine will take prices from a broader selection of sellers, including textbook warehouses and online auction sites.
Most professors will notate on their syllabus whether they will allow use of the previous edition. You will find that many booksellers will list plainly the differences between editions. Most times, there are minimal changes in the page numbers and little else. It may be worthwhile to buy a used edition even if you have to take a bit more time to find the right page. When searching online, you will find that many sellers will offer you the differences between the edition they are selling and the newest edition so you can make an informed decision.
Most sellers will include their shipping cost up front. Be sure to ask for a delivery confirmation number and print out your receipt for the book's purchase. If the books total with shipping is more than the used price listed on your campus bookstore's website, then purchase the book locally. This is particularly true for importing books as well. Many students find that buying books overseas (India is particularly popular) and having them shipped to the US provides for much cheaper textbooks. However, shipping can take nearly a month, so be prepared for the delay.
Ordering your book in advance will assure you the best prices and that the book will be in stock from the seller. Send an email to your professors in advance of the semester to inquire about the books. If the professor or the book is unassigned, then call the campus bookstore and inquire as to the title. Most college bookstores solicit the book titles at least four months in advance of the semester's start.
A little research will prove beneficial for finding the best prices. Some organizations, such as TextbookRevolution.org, are student-run organizations that let other students know about free resources and online textbooks that won't cost you a dime.