It perhaps goes without saying that the college book stores do not give very good prices. 20% is not unusual. If you're luck, some stores like the Follett run stores will, and certain books they know will be used in future semesters, ave a 50% guarantee for buy-back, but those are few & far between, and sometimes have other restrictions attached.
From what I've noticed, Amazon seems to have the best options for this, considering their main market and the sheer numbers of people looking for books here. They seem to sell for higher than ebay, and students shop here more often because a wide selection is guaranteed
Ebay prices will probably tend to be a little lower, since a lot of students don't look towards ebay when searching for their text books, since it is often difficult to find all of the books you need at one time, and you have to go through te bid process and hopefully win, or start all over again.
Other students at your campus may be a good source, since they'll be taking the same classes with the same curriculums. This probably requires more leg-work though, in putting up signs trying to find buyers, etc.
One final option are book swaps. Some college student organizations run these, or something similar, where you can swap your old text books for someone else's old text books that you need. This won't get you any money, but it will mean you don't have to spend money for the new books you need.
Good Luck!