This is a great and simple exercise to strengthen the lower back, abs and glutes.
Lie face down with your legs together. Place your hands by the side of your head, keep your shoulders relaxed and your core engaged.
Slowly lift your upper body off of the floor controlling the movement with your core. Nice smooth movements with no jerking. Keep legs on the floor.
Once your at the top of the movement hold for a couple of seconds before slowly lowering back down to the ground. Keep your core engaged throughout the movement.
Aim for between 5 and 25 repetitions. You can always start with low repetitions and build up as your strength improves.