The iPhone 6 is out tomorrow, and plenty of people are excited about it. So much so that they're planning to - or already have - traded in their old iPhone to go towards it. The thing about trading in hardware is it's very important to make sure your personal information isn't still on there once it leaves your hands. You generally have nothing to worry about when going through reputable businesses, but these days it never hurts to make sure.
Chances are you'll have to wait a bit if you didn't already preorder your iPhone 6, and in the meantime you'll probably be updating to iOS 8. If so, you'll want to know how to clear all the data from your phone with the new iOS before you hand it off for a downpayment.
As with all things regarding sensitive data, you'll want to start in your phone's Settings menu (I apologize for the mess on my home screen, I wasn't expecting company).
And that should do it! Your iPhone is now clear of any sensitive/incriminating information and you may trade it in without fear of a stranger stumbling upon that picture you took at that party that time.