So you want to sell your iPhone on Craigslist, Ebay, or wherever, but you're worried about someone seeing your contacts, old pictures, etc.?
Here's a good way to make sure that no one will be able to retrieve your old data.
Basically what you are doing is a an overwrite, 3 times. Get ready for it to take a long time, but your privacy is worth it, right?
You can also try this
Go to Settings - General - Reset - Erase All Content and Settings