How to clear Android Phone before selling it?

Perhaps you thought about or going to consider one day sell your Android phone, and of course you might clear your images, applications, as well as accounts of the device, in order to ensure that browsed the other party? But have you ever wondered to what limit these data are protected now? Can they be retrieved after?
Of course smart phones now become combines many of the sensitive data, which are directly related to us, as well as including us, but removing those files in the traditional way, does not mean they are protected 100% ,because with using some apps , it will be easy to retrieve files and data stored in your phone for a period of more than a month on the day of removal!! Thus, before thinking about selling your Android
phone you must consider this issue.

Do not ask yourself whether the buyer would be interested in you files and data but also do NOT ASK What he can do with them, but Ask yourself how i am gonna protect my files and data after i sale my phone.
In addition to set the phone to factory settings where this can be done easily by going to:
 

Android 4.0 and above
Settings >> backup and rest >> personal data >> Factory reset

on Android 2.3
Settings> Privacy> Factory data reset Download any of the following applications and clear your storage space , applications working to full storage space with random data in order to destroy your data before
clearing it again which makes retrieving it by any other program is something like impossible.