This is a deep topic: physiology, kinesiology, biomechanics. There are also several theories of movement: Alexander, Feldenkrais, Laban, Ideokinesis, etc. All of these theories approach the body as an interrelated mind/body system. Sound skiing is based on a wholistic integration of posture analysis, movement tendencies, boot selection and fit, adequate footbeds, skier goals, and psychological make-up. Rarely can you find all this in one person. So, more often than not, you've got to tinker, using the experts you can find. If you race, you may have a coach who is good at sizing all this up and suggesting solutions and people to see, if you need to see experts.
Another issue is that posture and alignment can change, if you exercise, train, and work on your body with some of the above-mentioned systems. Most people have posture, alignment and movement issues--and it's not their fault. It's just the way they are. And there is no one sytem to correct everything and keep it corrected. Bodies are always changing.
A good book that talks about movement theories and then presents a rather exhaustive set of movement mind/body exercises is Eric Franklin's "Dynamic Alignment Through Imagery." This is not a quick fix book, but dancers, movement specialists, and sports trainers the world over use it. Most of the folks on the World Cup have worked with experts in the above-mentioned areas. Many of them get the best fitting and re-fitting of boots. Plus, they get the best footbed consultation. And they select the right flex skis for their body types, weight, skiing style, etc. They also practice a lot. They all look great when they ski, even though they, too, make mistakes and fall and crash. But, as one of my friend, a dancer and movement educator told me, they all look great skiing through the gates.
The key that they get is that they all work with coaches who develop them as individuals, and with experts who do that with their bodies. The problem with most skiing instruction is that it ignores individual and tries to fit you into a pre-existing pattern that may not really fit the way your body, mind and skiing work.
That's why people may outwardly mimic what they see a world cupper doing, but they ski very poorly. They haven't had the individual development that is proper to them.
Look around and check it out. Beware of a guranteed fix, but seek out sound advice from someone who is willing to work with you, refit boots, and tinker with footbeds. Also, seek the same in an instructor/coach. What works for me won't work for you, and vice-versa.
PS--I mention all of this becasue other sports have this at some level and skiing lags a lot for the average person who could have a much richer experience on the hill if they were focused on.