Broomstick Tilt
This exercise can be performed either seated or standing, however seated helps isolate the oblique muscles, and prevents lateral hip rotation. If standing have a wide leg stance, if seated aim to have your legs wide, feet flat on the floor.
Keeping
your hips pointing forward, slowly tilt laterally from the waist to one-side,
whilst holding a broomstick across your shoulders, arms extended straight
along the broomstick.
Begin with small movements, focusing on contracting the oblique's to the side that you're turning too. Gradually increase the range of movement, however always aim to keep your hips forward, utilizing the broomstick to keep your back straight, and to avoid rotating your shoulders forward.
A good exercise for beginners, aim to work for 10 - 12 slow movements each side, repeat again after 20 - 30 seconds rest.
Intermediates repeat as above, but at the final turning phase, hold for a split second prior to repeating on the other side.
Advanced, use the above method as a warm-up for harder abdominal exercises, or alternatively use a barbell rather than a broomstick. If using weights on the Barbell, insure that the weights are secured correctly with adequate collars.
Never force into movement, allow your oblique's to work in a slow contracted motion throughout. You may want to alternate sides, or work one side at a time (harder).