Basic swiss ball 1 - tilt, bump and hula
While I'm not a huge fan of replacing your actual chair with a Swiss ball, I am a fan of sitting on a Swiss ball to exercise. My sister is a helicopter pilot and - knowing I used to teach Swiss ball classes when I was in the Army - she once described learning to fly a helicopter as "like sitting on a Swiss ball with your legs and arms crossed, and your eyes closed" - ouch!
This exercise - tilt, bump and hula - is great for restoring pelvic flexibility and control. You're likely to be working more on flexibility when your feet are apart, as this increases the base of support - and more on control when your feet are together or when you've got one foot in the air!
But whichever position your feet are in, try to move as smoothly as you can.
Equipment wise, I'm using a 65cm anti-burst ball, from 66Fit, a company who make reasonably-priced equipment for physios. I'm 5'6" tall and this is the right size for me. If you're much over 6 feet tall, I'd recommend going up to a 75cm ball; if you're under 5 feet, definitely go for a 55cm ball.
Want to know more? Find me at nellmead.com or join my Facebook group