Turn off iMessage
If you're switching to a non-Apple phone and you forget to turn these off, your phone number will continue sending them instead of text or SMS messages. That's bad. Tap Settings > Messages.
Restore to factory settings
Tap Settings > General > Reset > Erase All Content And Settings. You may need to enter your Apple ID at this stage if you had Find My iPhone turned on, then tap "Erase." Shed a single tear.
When all else fails, erase it remotely
If you lost your phone, had it stolen, or just forgot to wipe it before trading in, you can easily erase it from afar if you've been using iCloud and Find My iPhone. Simply sign in to your account here, select the device in question, and click erase. Then once it's erased, hit "Remove From Account," Lastly, call your carrier and tell them to disassociate the device from your account, and breathe freely.
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Joe McGauley is a senior writer for Thrillist. He wonders who, if anyone, is using his long-retired 3GS right now.