Success is in the elbow and the wrist.
Keep your hands in view at all times. Lose sight of one hand, and you've probably over-rotated or counter-rotated your upper body out of position for the next turn. Think about how you touch your pole to the snow. When the skiing is easy, plant it with your knuckles forward (A). When the terrain is steep, icy or bumpy, plant it with your palm forward and your elbow closer to your rib cage (B). It's easier to stabilize (block) your upper body when your elbow is in tight.
When you find yourself out of balance or uncertain about what's ahead, use a double pole plant. Pull yourself forward off your heels, and steady yourself by touching both poles to the snow. It's not a good idea to use a double pole plant all the time though, because swinging both poles forward locks your upper body in a position facing the ski tips instead of toward the upcoming turn.
A good pole plant comes from the wrist swinging the pole tip forward. Keep your hand and arm relatively quiet, and let your wrist flick the pole ahead. Your wrist action should be faster in short turns and more leisurely in long turns. As you plant one pole to start a turn, immediately start swinging the other to get ready for the next plant/turn sequence. Pole swing should be continuous-if you hesitate between swings, your pole plant will be late, throwing you out of rhythm and off line.