Can somebody retrieve my pics and Contacts?!

[rayes83]

I switched to iPhone 4s and am planning to sell my old iPhone 4, but some
friends warned me not sell an iPhone because all the pictures, calenders and contacts can be retrieved by the new buyer.

Whats the possibilities that my info can be restored by somebody else?

And whats the best way to format my iPhone?

 

[hobbbz]

goto settings> general> Reset. it will be brand spanking new

 

[itsmemuffins]

Or just restore without a sim and not activate the phone.

 

[rayes83]

Wirelessly posted (Mozilla/5.0 (iPhone; CPU iPhone OS 5_0_1 like Mac OS X) AppleWebKit/534.46 (KHTML, like Gecko) Version/5.1 Mobile/9A405 Safari/7534.48.3)

Thanks for your help