Is not as easy as it sounds:

Resetting Google Android smartphones to the original factory settings doesn’t always work, [McAfee identity theft expert Robert] Siciliano says. “On iPhones and iPads, we found little to no data; BlackBerry, same thing,” Siciliano says. “Even when someone did a factory reset on an Android, we still found a tremendous amount of data.”

His solution: “I would beat the thing to death.” Ouch.