Step 1: Lie down on your back, with your knees bent and arms out to your sides. Place your right foot on top of a yoga block [or a stack of books, if you don't have access to equipment] and your left foot straight up in the air. Contract your abdominals to flatten your low back into the floor.
Step 2: While maintaining this abdominal contraction, exhale and push through the right foot to bring your hips up off of the floor. As you do this, contract your glutes.
Step 3: Lower your hips back down toward the ground, while inhaling and maintaining control using the right hip.
Do 10 reps of this exercise on each leg for 1 to 3 sets.
To regress this exercise, simply omit the elevation with the yoga block, and perform the exercise with your foot flat on the floor.