A lot of the muscular qualities that are important for downhill skiing are also important for other sports: single leg strength, posterior chain strength, hip stabilization, core endurance and strength endurance. So if you're existing training program focuses on these qualities, great. If not, I'm going to suggest five exercises to work into your training program that stress these qualities:
Low Box TKE. One of my favorite exercises in regards to knee health. Not only are you strengthening the quadriceps, but working from the unsupported position brings your hip and foot stabilizers into play.
Trap Bar Deadlifts. Posterior chain strength is crucial when it comes to maintaining knee and low back health. Plus the positioning of the trap bar deadlift looks close to that of the tuck position during a downhill run.
Low Cable Split Squat. Any split squat variation is great to work out any left to right asymmetries. Left to right asymmetries are a good predictor of future injury. Using the low cable position also brings in an anti-flexion and anti-rotation core component also.
KB Swings. Another posterior chain exercise. I'd suggest performing these in the higher rep range (20-25 reps) or for timed sets to increase the time under tension for the set and bring in more of a conditioning component.
Bulgarian Squat Holds. This exercise serves two purposes. One is to create that burning sensation in your legs you may get after a hard downhill run. Two, to help open your hips and increase your hip mobility. Poor hip mobility leads to knee and low back injuries. To perform, get into the bottom position of a Bulgarian Squat (some refer to it as a rear foot elevated split squat) and hold for 30-60 seconds on each leg. A great way to finish off your leg workouts.
You may not be ski ready now, but you can be if you get started on those exercises listed above. Do not be the one who is too tired to make another run, but be the one who wants to get that next run in.