Turning through the fall line too abruptly
Skiing is a matter of holding back, then letting go. Many advanced skiers have the holding-back part down, but suffer from fall line phobia: the fear of letting go when gravity causes their skis to accelerate and pull them downhill. When you turn too abruptly down the fall line, you make sudden turns that leave square-cornered Zs in the snow. It would be far better to leave round, S-shaped tracks behind you.
Be patient: let the skis turn you
Face the fall line with a calm upper body, and accept the idea that you're going to linger there a split second longer than you may be used to.
Change edges as you normally would, but do it softly. Use your feet and thighs to guide your skis toward the fall line.
Let your feet drift out from under you so your skis tip up onto a higher edge. Then be patient. Instead of rushing to turn your skis, give the skis a chance to turn for you.
As your skis swoop back out of the fall line, you can tighten your arc to control your speed (hold back). Let more pressure build up along the edge of the outside ski, and start to steer the skis (with your legs)- across the hill.
As you slow down, prepare to let go again as you re-enter the fall line. Once you trust your skis, you'll feel like you're in controlled flight. Letting go is the part of skiing that's most fun.