The single leg deadlift is an exercise that incorporates core and glut activation as well as balance, both of which are critical to good, safe skiing. It is important that you are able to perform a hip hinge properly before beginning this exercise. This will ensure that you can maintain a neutral spine and activate your glutes properly. http://ascentpt.blog/2013/11/06/exercise-of-the-week-hip-hinge/
To perform the single leg deadlift, balance on one leg and with the other knee straight, slowly hinge forward. Keep your spine neutral and in line with your non-standing leg. Reach the hand opposite to your standing leg towards the ground. Return to standing with control.
Do this on both sides. The goal is to maintain a flat spine and be able to reach the ground without rotating and tilting the spine. This will help increase your strength and stability, and will translate into more stable, solid skiing over time.