Target your posterior chain muscles. In this video from Men's Health, see a leg workout called the "single-leg squat touch down". This squat targets the muscles of the posterior chain, the glutes, hamstrings, and lower back.
Balance on your left leg with your knees slightly bent and your hands on your hips. Slowly bend your knee to lower your body towards the floor, while reaching towards the outside of your left foot with your right hand. Push through your hell and squeeze your glutes to return to the starting position.
This squat targets the muscles of the posterior chain, the glutes, hamstrings and lower back, as well as the stabiliser muscles of the legs and core. These muscles help prevent you from falling over. This is an ideal exercise for both beginners as advanced. Athletes should also try this exercise to help strengthen the back of the leg, which is often underdeveloped when compared to the quads at the front.
Too easy? Try super-setting this with a larger compound exercise such as the Dead lift to really work those muscles you can't see in the mirror.