Simply factory wiping your phone does not ensure your data is securely destroyed. iPhone Eraser can make sure all your data is gone, and cannot be recovered.

Comprehensive Apple Device Coverage

Despite being called iPhone Eraser, this software actually covers all iOS devices, including iPads and iPod touches. It can securely erase your data, ensuring that every trace of it is gone and cannot be recovered. This is ideal if you intend to sell your device and don't any sensitive information being seen by the new owner. Recovery is prevented by erasing all data and then writing to the storage to prevent the previous data being recoverable. There are different levels of erasing available, depending on how sensitive your data and how safe you want to play it. Multiple devices can be erased simultaneously, also.

Good App, Bit Expensive

iPhone Eraser is a great tool for ensuring your data is securely disposed of. You will need a Windows machine with iTunes installed to use it. And you will need the full paid for version (which is a bit expensive) to see it work, as the trial version barely covers the core functionality of the app.