Are Swiss workout balls (exercise balls) the best way to workout my abs?

First, most people don't realize how their core is involved in just about everything they do. It is what stabilizes your body. For skiing, your core is going to have to keep you stable while your legs and upper body are shifting/turning. This is why traditional core - even on the ball - is not going to serve you as functionally as you would like.

Doing unilateral movements is one way you can work your core. For example, when doing bench press, rows, dumbbell flyes, etc, use dumbbells and work only one side at a time. Trust me, going heavy with only one arm on a bench press means your core is going to be working double-time to stabilize your torso.

With your legs, squats and dead-lifts are fantastic for the core. With skiing on the agenda, you'll want to consider one-legged dead-lifts and squats (known as "King squats") which are key for stability in the leg joint as well as work the core as a peripheral muscle.

Twist crunches with hanging ab straps are another to look into.

Don't neglect your lower back - that is a major portion of your core. Hyperextensions and good mornings as well as deep squats and dead-lifts will help strengthen these muscles.