First, there is the external mechanical part. For skiers, an experienced boot-fitter can help you achieve proper alignment. It requires patience with boot selection, adjustment of the ramp angle inside the boot, possibly custom orthotics, cuff alignment and, perhaps, even canting of the boot sole. For boarders, it requires a knowledge of boot & binding selection, plus a setting up of stance angles and stance width.
The goal of each should be to permit your body to move fairly close to the axis of your natural skeletal alignment. The further you get from that, the more trouble you will have, since it will require your muslces to work in ways they have not evolved to do efficiently.
Second, there is the physique of your body. Proper movement or coordinated range of movement requires the development of strength, flexibility and endurance. These need to be assessed by someone with the correct training to see exactly what a particular individual needs to work on.
Anecdotally, most skiers and boarders need work on all three. Most lack the strength, flexibility and endurance to flex or bend their legs as technically required for higher level skiing or boarding.
Also, people can be taught to move properly and it can become "natural" to them.
In order to ski with your natural posture, it is essential that you find a centered stance through proper foot, knee and hip alignment. You should be aligned both laterally and fore/aft. This requires sound guidance for proper boot selection, fitting and, possibly, custom orthotics.
Notice the lines of force through each alignment possibility below. Click on image for skeletal schematic.
and Evrard
(Lines
Added)
Bow-legged or over-canted: Usually a wide stance struggling to get on one edge. Turns and transitions are clumsy and take time because of a complex sequence of bodily movements. Equal edge angles and skiing with both skis are difficult. Skiing takes a lot of energy.
Knock-Kneed or under-canted: Automatically on both inside edges, turns require rotation of downhill knee and thigh to the inside, placingknee in a vulnerable position, the so-called "A-Frame" of old-fashioned ski technique. This pushes your butt out back, which forces you to lean over at the waist and misalign yourself in an unstable position. You tend to steer one ski turns with leg rotation, not carve them by tipping both skis.
During the entire process, your boot fitter will be working to center you fore/aft, so you neither lean forward or backward, but can stand upright in yur boots and on your bindings.
Proper sizing and selecting the right boot for your foot, ankle and lower leg are a central component of fore/aft alignment.
On the snow, you will need to get a feel for the right amount of ankle flex you need. If you have the right boots for your body and are aligned properly, this should be fairly easy.
Click on images for a skeletal view of schematic of "backseat skiing."
Just Right
Flexed
Flexed Enough
But there is no general formula, since each person has a unique biomechanical structure and unique riding style.
Generally, it'll take some fiddling around on the hill to get it right.
Notice the upright and aligned posture of these two riders. These are good starting positions and alignments from which to assume your riding stance.
You should be able to go from this stance to a low crouch and back again without much trouble.
Snowboard School's Instructional Manual
Flexibility and Endurance
His knees track over the center of his feet. His back maintains its four natural curves, with his head erect, and arms overhead.
From a view from the front (not shown), both sides of his body are symmetrical.
From each of the side view pictures, he bears the weight of the bar in alignment with his frontal plane. Heels, ankles, hips and shoulders are in a plane vertial to the ground that permits the skeleton to bear the weight, while the major muscle groups provide the sort of locomotion they have evolved, over millions of years, to do.
The Ovehead Squat may not be an exercise for beginners to start with, but, with a reasonable exercise program with trained supervision, it can become a mainstay exercise for serious skiers and boarders!
Many of the exercises on the Fitness, Sensing and Imaging pages of this site will help facilitate better movement patterns that may assist in helping you to move better while you work on strength, flexibility and endurance for improved skiing and/or boarding.