This is an awesome way to relieve tension in your lower back and work your butt at the same time. (A cushy mat will keep your tailbone from crying afterward.)
Lie on your back with your arms at your sides with your knees bent and your feet on the floor.
Lift your hips toward the ceiling. Hold for 1 count, and then lower back down.
Repeat the lifts for 60 seconds, squeezing your glutes and hamstrings at the top of the range of motion. Be careful not to overarch your spine.
To make this exercise more difficult, extend one leg at the top of the lift. Keep your thighs parallel and hold the lifted position for about 5 seconds.
Keeping your hips up, place your foot back on the floor and then lower your hips.
Repeat this exercise for 30 seconds; switch sides and do the move for another 30 seconds on the other leg.