Depends on a few factors:
Staying with your current carrier?
Moving to a new carrier? = Sprint will price your phone model and give you a credit towards the purchase of your new phone.
Samsung was recently offering to purchase iPhones, if you purchase a GS3
Lots of places online buy your cell phone from you at a decent price.
Depending on how old your iPhone is, you might be able to sell it. I recently sold my friends iPhone 4s for $400 to someone off of Craigslist.
Delete and wipe clean all your data before trading, selling or giving it away!