Mountain Biking is Good for You
It almost goes without saying ... but summer training is vital for strong winter skiing and snowboarding. Even better than spending time in the gym is to find a sport as enjoyable (if that's possible) as skiing. I love mountain biking and I ride spring, summer and fall (and occasionally winter). While the Northern hemisphere waits for winter, here is a little late summer mountain bike inspiration.
SkiNet.com - Mountain Biking is Good for Skiers
LiveStrong.com - Ski Endurance Training