First you need to figure out what your textbook is worth right now. Go to amazon.com and do a search for your title. Refine the search so that you get just the specific edition that you are selling.
I finished a semester last fall where I had a 4th Ed. of MS Office Beginners edition and all 4 editions came up on my search.
Timing is everything:
When you have located your edition of the book make note of the used price. Now if you're looking at these prices at the end of the semester the demand will be low and the price will be at it's lowest. If you hold onto that book until just before the next semester starts demand will be much higher and thus you'll get a better price.
If you can wait that long. Just before the next semester post a for sale ad on craigslist.org for the textbook. Put the price about 10% higher than amazon.com's lowest used price. The reason for this is that the instant availability is worth paying 10% more to someone who needs it now.
If the book hasn't sold by the first day of the semester go on campus and post a flier on the campus bulletin board advertising the book for 25% less than the bookstore price (This should be an even higher price than what you listed on craigslist).
If it still doesn't sell by the end of the first week go to ebay and post an add for the book. Ebay should have enough demand for your title that it will sell at the best market price possible.
I've bought plenty of books on ebay but I've never had to resort to selling them back on ebay. This last resort method always leads to a sale.