7 Ways To Protect Your Smartphone From Hackers – #2 and #7 is highly effective

6 Wipe Clean a Lost or Stolen Phone
Not only is losing your cell a huge inconvenience that can set you back hundreds of bucks, it can also leave you open to attack if you haven’t secured your device. To protect yourself, activate the “Find My iPhone” or “Android Device Manager” app, which allows you to track your phone’s location and remotely delete all the data.

If you sell your old phone, be sure to reset it to factory settings and remove your SIM card and additional storage cards. Give the phone a thorough sweep to make sure none of your personal info is lingering.