Tip:
I love single leg squats. But there is a slight risk of injury to the supporting knee. Better to balance with one or two hands when you train them from stand.
… Lift one leg directly in front of you, parallel with the floor (leg should be straight). Bend the supporting leg 90 degrees and press back up. Hands should be used only for balance and NOT for assistance in returning to a standing position. Repeat the exercise on the other leg.
From the GymSports New Zealand November 2007 newsletter (PDF) – Coaching | NZ Gymnastics
Gymnastics Coaching.com 