I just graduated from college, here is my advice on saving money:
1. Don't buy it from the college bookstore (if you have one) thinking that you need all your books right away.
2. Check the books out from the library, or through inter-library loan (or whatever your system may be called). Our library allowed us to check books out for the whole semster. I saved several hundred dollars this way. (this approach works best with novels and not with reference books).
3. If you have to purchase your books, try seeing if other students are looking to sell their own books-- put up posters around campus, and on the classifieds on your college's website (if you have such a thing).
4. If you have to buy them, look at Half.com, bigwords.com, powells.com, bn.com, amazon.com, ecampus.com, bamm.com, etc.
5. Last resort, buy them from the campus book store.
Good luck!
Also, make sure to sell back any books you don't need anymore as quickly as possible... if they become an old edition their value drops quick!