Chair Bridge is a great exercise that strengthens the hamstrings, glutes, core and lower back. It is common for one side to slightly tilt, but with strength from your hamstring, glute and especially your core, you can work on keeping your body stable and centered.

Chair bridge

Lie on the floor with one leg bent and foot on the floor.
Bend the other leg and bring up the foot.  Lift the bottom in order to move the foot upwards.
Hold for 20 seconds and move to other leg.
Aiming for 6-8 repetitions