You cannot improve without changing something and the most important thing to change in skiing, to advance to expert are the movement patterns.
Wrong movement patterns are at the root of all bad skiing, once the equipment is sorted out.
When changing movement patterns, you need to be very careful, slow and deliberate with the practice. You need to make sure that you understand very well the new movement patterns and replace the old one.
It is often hard to unlearn an old movement pattern so the best way is to learn a new pattern with new cues and in time simply replace the old pattern.
Complex movement patterns should be de-sequenced into their individual components, each component learned or changed and practiced individually and then the entire sequence re-composed back.
It is important that you pay attention to the movements, the sequence and the feelings and/or cues to give your brain all the opportunities to learn.
Especially paying attention to failures is a great aid: simply stop after a failure and think it through. It is important that this happens quickly - within a second of the failure. The brain will analyze what happened and improve on its own.
Drills to promote new movement patterns in skiing should emphasize:
Here is a very good example of how to change a few of the basic movement patterns in skiing: