Whenever it comes time to get a new cell phone you, obviously, have to retire your old one. Once you have transferred the data from the old device to the new one, it is (probably) best to either trade in the phone for a credit or recycle it. However, much like getting rid of an old computer, you should be sure to remove all personal data from your old cell phone.

The linked article has an extensive list of pieces which you should be sure to remove in the event they contain private information. Of course, each device is different so methods of removal will be vary. The article also provides links to several utilities which you may be able to use to delete the data easily, but if not then you might have to remove the data manually.