Save any pix or anything from the device onto your computer.
Make sure any games you have saved your progress on are somehow saved somewhere (sorry, don't know how to help you with that.)
Save all your contacts in gmail.
Remove your google account from the device.
Then I would use the setting to remove the sd card,
Factory reset from within the OS.
When booted back up, power down and remove sim.
Boot into recovery and clear the cache and factory reset again.
Reboot from recovery and check the device, it should be totally wiped.
Edit: hope I didn't miss anything. If so, I'm sure someone can add to my list
