1. Shop around
Compare prices on a variety of different websites. Find out what your classmates use and recommend. I tend to buy my books from Barnes & Noble because A) I trust them, B) the pricing tends to be relatively low in comparison to other places, and C) my shipping is free. A good way to keep costs down is to rent textbooks instead of buying them, though I prefer the option of reselling mine at the end of the year. Also, if you're looking at other sellers on a website like Amazon, watch that they don't run out of stock before you place the order.
2. Remember to take tax and shipping costs into consideration
When you're comparing prices and coming up with an estimate, don't forget to take tax and shipping into consideration. I like to know about how much my textbooks are going to cost, and there was one semester where my estimate was a lot lower than the actual cost because I forgot tax and shipping.
3. Buy from classmates
Another, generally lower cost option, is to buy used books from classmates who have already taken the class.
4. Know if there are required online codes or software
Some classes require textbooks that come with an online code, CD, or other software. If you have a class like that, be careful that any textbook you order comes with it. Some of these codes can only be used once and, therefore, require that you buy a new book.
5. Consider e-reader editions
E-textbooks are sometimes cheaper if you have the device. Be aware, though, of whether or not the professor allows e-textbooks to be used in class. I've had professors who didn't allow us to even have our laptops out during class and may have frowned upon e-readers as well.