Turning in old phone for new one

We have signed new contract with Sprint and in order to get the deal we want we need to turn in two old phones.

My concern is all of the info on the phone. We want to transfer pics/numbers from the old phones to the new ones but we also need to make sure the data is erased properly before we turn it over. Any tips to make sure our info (bank info, passwords, etc) is safe?

Turning in old phone for new one

restore your old phone to factory settings after you transfer