Verified answer

Go though the settings and do a factory reset. Either that, or do it from iTunes.

The only way that someone could recover your data would be to take the phone apart and do some jiggery-pokery with the memory. Most do not have that level of ability and those who do are not likely to destroy an iPhone just to get your music and contacts list.

I have executed this sometimes before. Plug your mobilephone into your pc and open iTunes. As soon as in iTunes your mobile will have to pop up and on the screen there will have to be a button that says restore. Click on on that and it is going to fix your mobilephone to its manufacturing facility settings. This will take a couple of minutes. Hope this helps