Although everyone gave good answers, most answers address the finer points of skiing. Most answers do not apply to the people that need to learn the basics.
Perhaps I am too practical, but when I look around the hill, I mostly see people in the backseat fighting to turn the ski or afraid to release out of the current turn. To these people balance, core stability, intent, quiet upper body, ski with your legs, be committed down the hill, dynamic anticipation, etc, means nothing.
The single best answer I heard so far is the release. This would help a beginner get out of the current turn.
But based on what I see out there, learning to stay out of the backseat and keeping the ski tips engaged and functional is the most important. Release is next. Bye the way, when I say "learn to stay out of the backseat" I don't mean for-aft balance. I mean getting over the fear to engage those tips and realize that once you do that the ski almost turns by itself.