Initiate shoulders before legs.
Mmmmm ... without wishing to get controversial, this is where bad habits sometimes start - it certainly did for me Whilst I concede that CASI do teach upper body rotation to beginers in order to get them started, intermediate turns avoid this as it introduces counter-rotation.
Although I do think that a degree of 'opening up' & 'closing' of the shoulder for turns can help - it certainly helped me with my switch riding.
Foot steering is definitely key though - especially for progression to riding in powder and steeper terrain.
I can definitely recommend Neil McNab's book 'Go Snowboard' which comes with a well made DVD - available from Amazon etc featuring the lovely Jenny Jones!
Having a lesson with a good instructor will mean they can observe your riding and make suggestions to improve your technique accordingly. We are all different and often require little tweaks here and there to help us ride with flow and grace.