I second the Addall suggestion. Here's what I started doing to save on books:
1. Buy books from bookstore that aren't shrink wrapped, meaning I can return them.
2. Order books from cheapest place at Addall.
3. Use books from bookstore until Addall books arrive, usually a few weeks because they'll have to be ordered.
4. Return books to bookstore, and use Addall books the rest of the semester.
You have to be very wary of the last date you can return books. Plus, the books might not arrive before this date. It's tricky, but you can save a Hunk of change, considering I've found books on Addall for 30-40 dollars cheaper than the bookstore.