Winter Training, should involve alot of core training...ie abs,lower back,ect... Try and find a friend who is a physcial therapist. Not a personal trainer, There is a major difference. Show them what wakeboarding is all about (Video-Pictures). Let them develop a plan for training the muscle that support the knee and ankle area. Remember that the number 1 injury in wakeboarding is Knees, with ankles being #2. Also, the use of a Bosu ball is great for balance....Then the trampoline, is a great tool for learning how to be upside down and developing muscles missed. But get the Bounce Board for that... Try a few trick on land.. Surface 360 (Great for learning the handle pass) or Surface 180 (Great for Hip to Rope switches)
These are a few thing we recommend for winter training at
www.learn2wakeboard.com Best of Luck