This may be posted a bit too late to help most, sorry, it just occurred to me to write this up, and I will give the reason for that later. I am sure some of you out in the intertubes haven't bought textbooks yet, and anyone else can file this away, if useful.
Every semester, as sure as anything, come out a flurry of articles and posts about the costs of books for school these days and every year they somewhat baffle me. The average cost is now listed as 1,1367$ per year, but in three and a half years in college and 1 1/2 in grad school I have never spent more than 200$ a semester and usually less than 100$. Now I have studied in cheap fields, textbook wise (although I was a psych minor and psych books are expensive), but that figure is the national average, not the average for Science majors only. Anyway, here is what I know.
Rule #1: Never ever, ever,ever,ever, ever shop at the campus book store. Even the used books are way more expensive than you can get elsewhere. There are no other rules, the rest is strategy.
Textbooks vs. monographs vs workbooks: a definition of terms
Textbooks, for this discussion anyway, are large usually multi-authored works meant to be an encyclopedic overview of the established facts of a certain subject. They have bland titles like "Introduction to Calculus" and "Data Security in a New Millennium". These are the most expensive new, but there exist a few strategies for dealing with them. Monographs on the other hand are single or (not technically, but for this discussion) team authored works which present a specific, possibly contested, point of view on a very focused subject and have titles like "Subjects of the Sultan: Culture and Daily Life in the Ottoman Empire" or "Double Fold: Libraries and the Assault on Paper", so dramatic/artistic vague title: explanatory, usually long sub-title. The good news is that they can usually be gotten cheaply. The bad news is that classes that assign them usually assign like six of them. Workbooks generally are median expensive and have to be bought new, so really you pretty much have to eat that cost. Sorry.
Getting Books for Free
The Interwebs: Your ability to get your book for free (legally) on the internet is substantially dependent on your discipline and almost exclusively limited to monographs and novels. If you are in an English class for pre-20th century lit, you can get your books for free (except any textbooks). Certain disciplines, like library/information and computer sciences are offering more and more monographs free online, and some academic publishers like Yale, University of Michigan, and MIT are posting a substantial number of new monographs. To find these, for books published before 1928, go to archive.org or Google Books. Otherwise, google the full book title and look for the book's website or page hosted on the publisher's website. If it's available there will be a link somewhere in there, usually obvious on the front page.
The Library: Your school library should have a copy of any of your textbooks on reserve. If they do not e-mail the library or your professor and, unless the library is completely flat broke, they will get a copy and place it on reserve and you can read it for free anytime. Of course, if you are in a very large class there may be competition for access to the reserve copy and you may just want to buy it. For the love of god don't try and request your text books from Inter-Library loan. Yes, this is why I was reminded to write this. I work for a small-ish academic library in Pittsburgh and part of my job is processing incoming ILL requests and I just finished rejecting a dozen (literally, that's not an exaggeration) requests for a basic computer security textbook from students at Texas A&M. Understand that the system sends requests first to libraries in the closest geographic vicinity and you begin to see the full ridiculousness. Here's the thing: every library has to put the needs of their patrons first, so if a textbook is commonly assigned and we own it, we probably own it because it is assigned for a class in our university (textbooks are both expensive and frequently stolen, so libraries don't generally buy them for general use, but for specific classes) and is therefore probably on reserve or checked out to one of our students. Also, if you are in one out of four sections of a class with 200 students, it should be obvious, even to an aggie*, you can't all borrow your textbooks. Occasionally you may be able to get a textbook if a university isn't taching that class for a given semester or has changed the assigned text, but that just isn't going to happen 200 times; you will just be super annoying to library interns across the country. You can use ILL to get monographs and novels from other universities, just not textbooks. Remember that ILL usually only allows you to borrow a book for a month or so, so don't request them right at the beginning of the semester if it is the last book you need to read.
Getting Books for Cheap
Aggregated Search Engines: The best deals on books are usually found on used sites, so I used to search through five different sites comparing prices. Lo, then did the aggregated searches appear, and the peasants rejoiced. This is the great secret to cheap books. My favorite is
Deal Oz, but if anyone else has a favorite drop it in the comments. Just keep combined shipping possibilities in mind. Also, if your assigned textbook is brand new, be sure to ask your professor if an older edition is okay, because usually it is and superseded editions are dirt cheap.
Textbook Rental: Sometimes these are a great deal, but I find they are rarely much cheaper, and sometimes more expensive, than a used copy. My personal opinion is that they are really only ideal for something like a required class outside your major or field of interest. If it is the introductory class in your major then chances are the textbook will be useful to keep around for future reference. I might just be a book hoarder though.
*Sorry, I went to High School in Austin. It's a thing
through your professors, and all of your classmates are potential future colleagues. This last consideration is especially noteworthy to me as, in the long run, I plan on getting a PhD in my field and Library/Information science is a teeny-tiny field where everybody knows each other. I would argue that none of this suggests you ought to try and hide or downplay your subcultural associations. After all, when the person you are meeting is someone who, in a few years, you legitimately could be skyping with at 2 in the morning to discuss a conference poster for a project you collaborated on, you really need to consider how long term you are willing to hide your spooky self.* Also, a semester is a long enough period of time for anyone who comes in with negative stereotypes to realize you are a srs bizness academic. For myself, while I feel no need to tone down any gothiness, I do take care to be neat, modest, and, at least at first, avoid any sexualised elements (like corsets, even over a blouse which would be perfectly modest) and really cliche elements or to keep them in highly stylized version (ie: this skull necklace, not this one). 
I have never been much of a shoe person. I think I have simple desires in shoes only to spend three weeks tracking down a pair that I like and that fits my (apparently) comically large size 11/42 feet. The net result is that I have one pair each of: sneakers, mary janes, ballet flats, insensible heels, rain boots, and regular boots that get worn as called for until they die. My desires for a pair of boots are that they are: real leather, black, matte-finish, slightly girly, pointy, low but some heel (ideal for bus-aisle surfing), and preferably Victorian/granny style. For some reason everything in my size is either a combat-style boot* or teetering on a straight-up dominatrix heel. I am six feet tall and simply do not wear 5-in heels, or if I do it is as a specific act of aggression. It was therefore with great trepidation that I started my quest to replace the boots that had gotten a giant hole in their sole last winter. But then, something amazing happened.
