Like cosmo said, cardio is great. Also, light to moderate weight training won't hurt. Work your legs too, especially the knees. And don't try anything stupid, like a trick way beyond your capability.
Like Kyle said too, if it is your time to get hurt, you'll get hurt no matter what preparations you've done. Seems like I hear of many more wakeboarders getting hurt than wakeskaters.
Just have fun & don't worry about the injuries.