I usually use comegetused.com; my favorite source although it's pretty unreliable since the site always seems to be down. It's great when it works though. You just meet up with sellers (students) and pay cash for the books, no shipping or tax.
If that fails, amazon and half.com are quite nice. Cheap and fast
Get the book when you are sure you won't drop the class. Sometimes there are certain classes that use the same book every year because they are good books (physics 110a always uses Griffiths Electrodynamics), and in that case feel free to get it earlier. Just don't get them too early because there's a chance they could publish a new edition by the time you take the class.
comegetused is a good site,
i agree with the 2 weeks into class comment.
and honestly, you can get by without buying books, because there will be multiple copies reserved at the library and because a lot of students buy their own books, these copies are pretty useful even during exam times... you should wait to see if you can get older editions (for most science classes) for cheaper price...
I like to buy older editions of textbooks especially for engineering classes. Most often, the professor will put up the problems online, or you can go to the library and look at the problems. This saves a significant amount. I usually findout before hand what the books are before class starts by emailing the professor/looking on the online schedule/asking other students if the class consistently uses the same books. If I am not sure, I can wait a few weeks into class.
textbooks price are very seasonal (supply and demand). It goes very cheap right now on half.com or other online website, and it get more expensive toward start of sem.
I buy textbook ahead of time when I am sure what class I need to take (major classes).
Last time, I was able to make money by selling textbook. I bought my new mcb 100b textbook before christmas for about 70$ online, then I figure there's not much different between the old edition, so I sell it back two week into the sem for $100.
For many textbooks, I am able to sell it back for about the price I bought it, so basically I am not paying anything to use the book. (i only keep books for my major)
I rented two of mine from the student store. IF your turn them back in with the same condition, it was basically free. I bought a pack from Ned's because that was the only place that had both books together and was cheap. usually, i get it off of Amazon but sometimes the price is about the same with shipping. SOMETIMES. so if you're in a hurry, make the comparison.