i am a huge proponent of yoga, especially core power yoga. just doing the stretches help, but you should really try to spend an hour in a class 3-4 times a week. www.corepoweryoga.com
has progressive, faster paced, heated yoga instruction that is a great workout and significantly improves balance and flexibility. they have locations in denver, boulder, and now other cities across the us. I have gotten away from it over the past 6 months because of limited free time and i can certainly notice it. it really helped with my skiing.