Factory reset (in your setting menu, probably under Privacy, but may be somewhere else depending on OS version). Take out your SIM card, if it has one. Remove your SD card. That should do it. Double check by looking in gallery to make sure it is clean and try gmail to make sure it requests your account data instead of logging you on. 

Commercial business that purchase used cell phones often claim to wipe them, but I wouldn't want to depend on that.