the best cross training for skiing, is ABSOULUTELY biking. it gets you in awesome shape, boost your endurance out the roof, and helps your balance immensely. just cause your not sliding sideways, it still makes a huge difference. plus its insainly fun. dont limit it to a specific genre either, ride some road, freeride/DH, jumps, XC, trials, they are all super fun, and have their benefits
skateboarding is a good one, as is rock climbing.