Bridging
Lie on your back with your hands by your sides, having both heels resting on the top of a Swiss Ball, with legs bent to 90 degrees.
Exhale and slowly contract your abdominal and back muscles, whilst straightening both legs to establish a straight line through your heels - hips and shoulders; lifting your lower-body with the weight on your shoulders, not your neck / head.
Aim to hold the extended position for 5 - 10 seconds prior to inhaling and lowering yourself back down; whilst you bring the ball back towards you.
Beginner's aim for 6 - 10 reps in a slow steady motion.
Intermediates aim for 2 sets of 8 - 12 reps in a slow steady motion.
Advanced work for 2 - 3 sets of 12+ reps, aiming to hold the extended phase for the full 10 seconds.
This exercise can be made harder by only having 1 foot on the ball, which reduces your balance; likewise keeping your hands off the floor will make the exercise harder.
For best results, work with a small firm ball.