For ski season preparation, start training for maximum ski performance and minimum injuries. The best ski/snowboard conditioning regimens encompass much more than just strengthening the legs. Many of the recommended ski/snowboard workouts out there jump straight into training the muscles of the lower extremities (legs) along with agility drills and plyometric. While these things are indeed important, it all starts with the CORE. Regardless of the sport or activity you might be training for, the core is the fundamental piece that must not be overlooked. Think of the core as everything between the shoulders and hips on all sides (front, back, and both sides of your body). Increased stability, endurance, and strength in the core (in that order of importance) will increase your athletic performance on the hill and decrease the likelihood of injuries to the knees and other joints. Your core should act as your anchor (practicing stability) from which you develop and project force via the hips and shoulders. Endurance in the core is important because you need to maintain good posture as you go down (or up) the hill. Endurance is the ability to maintain contractions in the core muscles that hold you upright as you ski/snowboard.
To start building endurance in your core, practice these exercises daily a week or two before you move into lower extremity strengthening and plyometric and then keep them in your regular routine.
Side Planks: 5 reps, 10-20 second holds each rep
Planks: 5 reps, 10-20 seconds each rep
Bird Dog: (opposite arm/leg extension): 10 reps, 10-20 second holds each rep
Crunches: 10 reps, 10-20 second holds each rep
REPEAT ALL ONE MORE TIME. Even if you can hold these positions longer, it is safer and more effective to get a good contraction for 10 to 20 seconds for each rep rather than holding the rep for a full minute and relying on the joints and ligaments to sustain you after the muscles fatigue. I like to do these in the beginning of the workout to turn the core muscles on and remind myself to keep a stable spine.
From there, you can move on to the traditional ski/snowboard conditioning exercises such as squats, jump squats, box jumps, agility drills, etc. I also recommend incorporating high intensity interval training (HIIT) into your workouts to really boost your fitness level.