Bring the textbooks you are done using to your campus bookstore. They may be able to take them back and give you a partial refund.
Donate your used textbooks to any store that sells college textbooks. You will get a tax refund from this.
Post your available textbooks on the web. There are different websites available for this (see Resources).
Use websites that post college textbooks specifically for trading. Some are set up so that you're trading with other students on your campus (see Resources).
Display a note in your student lounge advertising the textbooks you wish to trade.
Leave a list of the textbooks you need with your trade list to see if you and another student can make an even swap.
Get together with your friends in between every semester to trade textbooks. You may each have a book you're done with that the others will need in the upcoming semester.