how to associate more than one device to itunes, ie, ipod & iphone

Hi - i've recntly bought an iphone after having an ipod nano for a couple of years.

My music syncs great with itunes, but now does not recognise that my ipod nano is associated with my itunes.

Any thoughts, as I like to be able to sync music on both devices with my one itunes account.

Many thanks in advance...