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.
iOS Erase
Step 1:
Sign out of iCloud and the iTunes Store.
Step 2:
Tap 'Settings'.
Step 3:
Tap 'General Reset'.
Step 4:
Tap 'Erase All Content and Settings'.