A lot of kids make do with old editions. Not enough to make up the bestseller gap, though.
I really dislike using eBooks because reading on the computer hurts my eyes and I usually like holding the book in my hands when I'm making notes. That said, bigwords.com, as mentioned before, really helped me out last semester. I also know that some sites let you rent textbooks for a semester, which is cool if you're like me and don't write/highlight in textbooks at all.
Also try buying international editions: they're a lot cheaper ($30 on the ones I've bought, as opposed to $100+) and they are literally exactly the same on the inside. Just make sure you're buying the right edition and whatnot. The only difference for the ones that I've had is that they were paperback and don't look as nice, but that's really nothing compared to your savings. Also, you probably won't be able to sell it back to your campus bookstore but eBay's always good for that, too. So basically what everyone else has said. I guess I should add to that, as a general note for everyone who''s attempting to buy textbooks...
See if you can buy/borrow textbooks off of your friend who've already taken the class, or if the older editions of a book are similar enough to the newer edition to get by. Some books (like our Calc. I/II book) were pretty much exactly the same, except some of the problems were numbered differently. Also, be wary of classes that have no books listed: if they're art history/English/other arts and sciences courses, they're likely to bust out a syllabus with 10+ books on the first day. Not textbooks, but normal-ish books each in the $10-$30 range that can really run up your costs, even with eBay and Amazon. Make use of your school's/county's public library system in this case-- check out the books before anyone else does. And be a jerk and keep renewing them for as long as you can, like I do.
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Where can I download textbooks?