Go to options on the device and then in the advanced security options there should be an option to wipe all information on the device. You may also want to double check that you don't have an SD card still in the phone, and if there is and you intend on keeping it there then you may want to format it to wipe any information or pictures stored on it.
Also, you probably want to go to your BIS website and delete the device association with your email accounts, which will release it to be set up with other email accounts.
