My #1 piece of advice is to not buy books before you attend the first class, that way if the teacher isn't using all of the books, or using them 1 at a time if there are multiples, you don't have to get them all at once, and you don't buy ones you won't use.
Except you might get the professor who is trying to prove a point and docks anyone who shows up the first day of class without their required books (yep, had it happen - started out that class with negative points)
It also limits your options. You might not be able to order from some of the online discounters since you have to allow for shipping time.
I do agree that you should make sure you really need the books first though, but most of the time you can find that out just by talking to someone who has taken the class or by contacting the professor in advance.
DD needed 3 big expensive books this semester.
1. Found used (barely) on e-bay for about 1/3 the cost of buying new. Had it 4 days after payment was made.
2. Got from e-campus new for about 1/2 what the bookstore was charging. It did take 2 weeks to get here though.
3. Couldn't find a big enough savings anywhere to worry about so DD is buying at school.
I knew about Bigwords, but I'll have to check out campusi too. Sounds good
I've ordered online before only to get notice 2 weeks later that the book had not even been received in their warehouse yet.

Great - I was able to cancel, but left me scrambling to find something last minute when I thought I was ahead of the game.