Restoring contact list from old iphone 3G(3.1.3) to 3GS

Hi

I have an old 3G iphone(running 3.1.3) which I want to take all my contact list information to my second iphone which is 3GS running 6.0.1.

What should I do to transfer the contact list from my 3G iphone to my 3GS iphone?