This article outlines a method to ensure that data on your iOS device is rendered inaccessible once it's erased. iOS is encrypted by default, so a factory reset on an iOS device will suffice for a secure erase method.
Important! The following process will render any existing data on you device unrecoverable. Please ensure any data you wish to retain is stored elsewhere before proceeding.
iOS Erase
Step 1:
Sign out of iCloud and the iTunes Store.
Step 2:
Open your "Settings" app.
Step 3:
Tap "General Reset".
Step 4:
Tap "Erase All Content and Settings".