lots and lots of lunges, forward lunches, step back lunges, chair lunges, side step lunges, toe roll lunges, sneaky lunges. start incorporating weights into the lunges after a while. also do deadlifts and squats. i also do cardio like eliptical or stationary bike rides for 45 minutes. i usually do 1 day of purely leg exercises, 1 day of cardio and 1 day of yoga as my ski exercise specific days. i also do a day of arms and shoulders and a day of chest and back.
dont do core exercises (or any single group of muscles for that matter) every single day, do them 3 days a week max. if you do them every single day your body will become complacent and use to the exercise, you'll plateau and will not develop more strength. a proper balance of working out (and thus actually damaging your muscles) and letting them properly recover and heal is what makes them stronger. damaging them everything single day and not letting them recover and strengthen does you no good.