Here's the techniques I used when I was in college -- of course, we didn't have the internet back then, so I'm sure better methods can be incorporated:
Knowing that used books were limited, I'd go to the book store as soon as possible and I'd buy as many used books as I could. Then I'd search the bulletin boards for an individual selling that same book cheaper. I'd also refrain from writing in the books or removing any plastic wrap until AFTER the first class (sometimes the professor changed his mind about books, or sometimes a mistake was made, or sometimes I decided to drop the class). IF I found the books cheaper, I returned them to the bookstore. If not, I had the advantage of having bought the cheapest book.
Sometimes I shared books with dorm friends; this, of course, is not the most convenient method, but when the book costs $75 and you're paying your own way . . . well, you make do, take turns, and stay up late sometimes.
Sometimes I checked books out of the library, renewed them as often as possible, and just expected to pay some fines. It was still less than buying the books. This was especially useful in English classes.
For English classes, I was able to buy novels at the mall less expensively than I could buy them at the college bookstore.
I loved it when the professor put the books into the library's reading-room, which meant you could check out the book, but you could not remove it from the room -- you had to do all your reading right then, right there. Usually professors did this only when they'd required multiple books for a certain class, and they only required 1-2 chapters of each book to be read. Sometimes I xeroxed those chapters; sometimes friends xeroxed them and I shared.
At the end of the semester, I never sold my books back to the bookstore. It went something like this: You'd buy the book for $75. They'd buy it back from you for $4.50, and they'd sell it as a used book for $60. These numbers are NOT exaggerated. I hope that the advent of the internet has forced them to be more fair in their buy-back prices.
Obviously, this was NOT A DEAL, so I did what everyone else did: I'd put up signs offering my used books for sale; everyone on campus searched high and low for these used books, and we usually sold them for about 50% of the new price.
